449 lines
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ReStructuredText
449 lines
20 KiB
ReStructuredText
=================
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libtorrent manual
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=================
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:Author: Arvid Norberg, arvid@rasterbar.com
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:Version: 0.15.0
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.. contents:: Table of contents
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:depth: 2
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:backlinks: none
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tuning libtorrent
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=================
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libtorrent expose most constants used in the bittorrent engine for
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customization through the ``session_settings``. This makes it possible to
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test and tweak the parameters for certain algorithms to make a client
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that fits a wide range of needs. From low memory embedded devices to
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servers seeding thousands of torrents. The default settings in libtorrent
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are tuned for an end-user bittorrent client running on a normal desktop
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computer.
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This document describes techniques to benchmark libtorrent performance
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and how parameters are likely to affect it.
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reducing memory footprint
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=========================
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These are things you can do to reduce the memory footprint of libtorrent. You get
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some of this by basing your default ``session_settings`` on the ``min_memory_usage()``
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setting preset function.
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Keep in mind that lowering memory usage will affect performance, always profile
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and benchmark your settings to determine if it's worth the trade-off.
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The typical buffer usage of libtorrent, for a single download, with the cache
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size set to 256 blocks (256 * 16 kiB = 4 MiB) is::
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read cache: 128.6 (2058 kiB)
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write cache: 103.5 (1656 kiB)
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receive buffers: 7.3 (117 kiB)
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send buffers: 4.8 (77 kiB)
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hash temp: 0.001 (19 Bytes)
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The receive buffers is proportional to the number of connections we make, and is
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limited by the total number of connections in the session (default is 200).
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The send buffers is proportional to the number of upload slots that are allowed
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in the session. The default is auto configured based on the observed upload rate.
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The read and write cache can be controlled (see section below).
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The "hash temp" entry size depends on whether or not hashing is optimized for
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speed or memory usage. In this test run it was optimized for memory usage.
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disable disk cache
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------------------
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The bulk of the memory libtorrent will use is used for the disk cache. To save
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the absolute most amount of memory, you can disable the cache by setting
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``session_settings::cache_size`` to 0. You might want to consider using the cache
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but just disable caching read operations. You do this by settings
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``session_settings::use_read_cache`` to false. This is the main factor in how much
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memory will be used by the client. Keep in mind that you will degrade performance
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by disabling the cache. You should benchmark the disk access in order to make an
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informed trade-off.
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remove torrents
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---------------
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Torrents that have been added to libtorrent will inevitably use up memory, even
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when it's paused. A paused torrent will not use any peer connection objects or
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any send or receive buffers though. Any added torrent holds the entire .torrent
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file in memory, it also remembers the entire list of peers that it's heard about
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(which can be fairly long unless it's capped). It also retains information about
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which blocks and pieces we have on disk, which can be significant for torrents
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with many pieces.
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If you need to minimize the memory footprint, consider removing torrents from
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the session rather than pausing them. This will likely only make a difference
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when you have a very large number of torrents in a session.
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The downside of removing them is that they will no longer be auto-managed. Paused
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auto managed torrents are scraped periodically, to determine which torrents are
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in the greatest need of seeding, and libtorrent will prioritize to seed those.
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socket buffer sizes
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-------------------
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You can make libtorrent explicitly set the kernel buffer sizes of all its peer
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sockets. If you set this to a low number, you may see reduced throughput, especially
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for high latency connections. It is however an opportunity to save memory per
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connection, and might be worth considering if you have a very large number of
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peer connections. This memory will not be visible in your process, this sets
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the amount of kernel memory is used for your sockets.
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Change this by setting ``session_settings::recv_socket_buffer_size`` and
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``session_settings::send_socket_buffer_size``.
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peer list size
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--------------
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The default maximum for the peer list is 4000 peers. For IPv4 peers, each peer
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entry uses 32 bytes, which ends up using 128 kB per torrent. If seeding 4 popular
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torrents, the peer lists alone uses about half a megabyte.
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The default limit is the same for paused torrents as well, so if you have a
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large number of paused torrents (that are popular) it will be even more
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significant.
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If you're short of memory, you should consider lowering the limit. 500 is probably
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enough. You can do this by setting ``session_settings::max_peerlist_size`` to
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the max number of peers you want in the torrent's peer list.
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You should also lower the same limit but for paused torrents. It might even make sense
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to set that even lower, since you only need a few peers to start up while waiting
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for the tracker and DHT to give you fresh ones. The max peer list size for paused
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torrents is set by ``session_settings::max_paused_peerlist_size``.
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The drawback of lowering this number is that if you end up in a position where
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the tracker is down for an extended period of time, your only hope of finding live
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peers is to go through your list of all peers you've ever seen. Having a large
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peer list will also help increase performance when starting up, since the torrent
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can start connecting to peers in parallel with connecting to the tracker.
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send buffer watermark
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---------------------
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The send buffer watermark controls when libtorrent will ask the disk I/O thread
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to read blocks from disk, and append it to a peer's send buffer.
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When the send buffer has fewer than or equal number of bytes as
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``session_settings::send_buffer_watermark``, the peer will ask the disk I/O thread
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for more data to send. The trade-off here is between wasting memory by having too
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much data in the send buffer, and hurting send rate by starving out the socket,
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waiting for the disk read operation to complete.
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If your main objective is memory usage and you're not concerned about being able
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to achieve high send rates, you can set the watermark to 9 bytes. This will guarantee
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that no more than a single (16 kiB) block will be on the send buffer at a time, for
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all peers. This is the least amount of memory possible for the send buffer.
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You should benchmark your max send rate when adjusting this setting. If you have
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a very fast disk, you are less likely see a performance hit.
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optimize hashing for memory usage
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---------------------------------
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When libtorrent is doing hash checks of a file, or when it re-reads a piece that
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was just completed to verify its hash, there are two options. The default one
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is optimized for speed, which allocates buffers for the entire piece, reads in
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the whole piece in one read call, then hashes it.
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The second option is to optimize for memory usage instead, where a single buffer
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is allocated, and the piece is read one block at a time, hashing it as each
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block is read from the file. For low memory environments, this latter approach
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is recommended. Change this by settings ``session_settings::optimize_hashing_for_speed``
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to false. This will significantly reduce peak memory usage, especially for
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torrents with very large pieces.
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reduce executable size
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----------------------
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Compilers generally add a significant number of bytes to executables that make use
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of C++ exceptions. By disabling exceptions (-fno-exceptions on GCC), you can
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reduce the executable size with up to 45%. In order to build without exception
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support, you need to patch parts of boost.
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Also make sure to optimize for size when compiling.
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play nice with the disk
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=======================
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When checking a torrent, libtorrent will try to read as fast as possible from the disk.
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The only thing that might hold it back is a CPU that is slow at calculating SHA-1 hashes,
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but typically the file checking is limited by disk read speed. Most operating systems
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today do not prioritize disk access based on the importance of the operation, this means
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that checking a torrent might delay other disk accesses, such as virtual memory swapping
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or just loading file by other (interactive) applications.
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In order to play nicer with the disk, and leave some spare time for it to service other
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processes that might be of higher importance to the end-user, you can introduce a sleep
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between the disc accesses. This is a direct tradeoff between how fast you can check a
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torrent and how soft you will hit the disk.
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You control this by setting the ``session_settings::file_checks_delay_per_block`` to greater
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than zero. This number is the number of milliseconds to sleep between each read of 16 kiB.
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The sleeps are not necessarily in between each 16 kiB block (it might be read in larger chunks),
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but the number will be multiplied by the number of blocks that were read, to maintain the
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same semantics.
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high performance seeding
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========================
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In the case of a high volume seed, there are two main concerns. Performance and scalability.
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This translates into high send rates, and low memory and CPU usage per peer connection.
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file pool
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---------
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libtorrent keeps an LRU file cache. Each file that is opened, is stuck in the cache. The main
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purpose of this is because of anti-virus software that hooks on file-open and file close to
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scan the file. Anti-virus software that does that will significantly increase the cost of
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opening and closing files. However, for a high performance seed, the file open/close might
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be so frequent that it becomes a significant cost. It might therefore be a good idea to allow
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a large file descriptor cache. Adjust this though ``session_settings::file_pool_size``.
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Don't forget to set a high rlimit for file descriptors in your process as well. This limit
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must be high enough to keep all connections and files open.
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disk cache
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----------
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You typically want to set the cache size to as high as possible. The
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``session_settings::cache_size`` is specified in 16 kiB blocks. Since you're seeding,
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the cache would be useless unless you also set ``session_settings::use_read_cache``
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to true.
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In order to increase the possibility of read cache hits, set the
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``session_settings::cache_expiry`` to a large number. This won't degrade anything as
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long as the client is only seeding, and not downloading any torrents.
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peers
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-----
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First of all, in order to allow many connections, set the global connection limit
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high, ``session::set_max_connections()``. Also set the upload rate limit to
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infinite, ``session::set_upload_rate_limit()``, passing 0 means infinite.
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When dealing with a large number of peers, it might be a good idea to have slightly
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stricter timeouts, to get rid of lingering connections as soon as possible.
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There are a couple of relevant settings: ``session_settings::request_timeout``,
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``session_settings::peer_timeout`` and ``session_settings::inactivity_timeout``.
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For seeds that are critical for a delivery system, you most likely want to allow
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multiple connections from the same IP. That way two people from behind the same NAT
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can use the service simultaneously. This is controlled by
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``session_settings::allow_multiple_connections_per_ip``.
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In order to always unchoke peers, turn off automatic unchoke
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``session_settings::auto_upload_slots`` and set the number of upload slots to a large
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number via ``session::set_max_uploads()``.
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torrent limits
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--------------
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To seed thousands of torrents, you need to increase the ``session_settings::active_limit``
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and ``session_settings::active_seeds``.
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benchmarking
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============
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There are a bunch of built-in instrumentation of libtorrent that can be used to get an insight
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into what it's doing and how well it performs. This instrumentation is enabled by defining
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preprocessor symbols when building.
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There are also a number of scripts that parses the log files and generates graphs (requires
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gnuplot and python).
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disk metrics
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------------
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To enable disk I/O instrumentation, define ``TORRENT_DISK_STATS`` when building. When built
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with this configuration libtorrent will create three log files, measuring various aspects of
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the disk I/O. The following table is an overview of these files and what they measure.
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+--------------------------+--------------------------------------------------------------+
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| filename | description |
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+==========================+==============================================================+
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| ``disk_io_thread.log`` | This is a log of which operation the disk I/O thread is |
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| | engaged in, with timestamps. This tells you what the thread |
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| | is spending its time doing. |
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+--------------------------+--------------------------------------------------------------+
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| ``disk_buffers.log`` | This log keeps track of what the buffers allocated from the |
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| | disk buffer pool are used for. There are 5 categories. |
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| | receive buffer, send buffer, write cache, read cache and |
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| | temporary hash storage. This is key when optimizing memory |
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| | usage. |
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+--------------------------+--------------------------------------------------------------+
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| ``disk_access.log`` | This is a low level log of read and write operations, with |
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| | timestamps and file offsets. The file offsets are byte |
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| | offsets in the torrent (not in any particular file, in the |
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| | case of a multi-file torrent). This can be used as an |
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| | estimate of the physical drive location. The purpose of |
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| | this log is to identify the amount of seeking the drive has |
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| | to do. |
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+--------------------------+--------------------------------------------------------------+
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disk_io_thread.log
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''''''''''''''''''
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The structure of this log is simple. For each line, there are two columns, a timestamp and
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the operation that was started. There is a special operation called ``idle`` which means
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it looped back to the top and started waiting for new jobs. If there are more jobs to
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handle immediately, the ``idle`` state is still there, but the timestamp is the same as the
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next job that is handled.
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Some operations have a 3:rd column with an optional parameter. ``read`` and ``write`` tells
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you the number of bytes that were requested to be read or written. ``flushing`` tells you
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the number of bytes that were flushed from the disk cache.
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This is an example excerpt from a log::
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3702 idle
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3706 check_fastresume
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3707 idle
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4708 save_resume_data
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4708 idle
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8230 read 16384
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8255 idle
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8431 read 16384
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The script to parse this log and generate a graph is called ``parse_disk_log.py``. It takes
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the log file as the first command line argument, and produces a file: ``disk_io.png``.
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The time stamp is in milliseconds since start.
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You can pass in a second, optional, argument to specify the window size it will average
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the time measurements over. The default is 5 seconds. For long test runs, it might be interesting
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to increase that number. It is specified as a number of seconds.
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.. image:: disk_io.png
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This is an example graph generated by the parse script.
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disk_buffers.log
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''''''''''''''''
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The disk buffer log tells you where the buffer memory is used. The log format has a time stamp,
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the name of the buffer usage which use-count changed, colon, and the new number of blocks that are
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in use for this particular key. For example::
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23671 write cache: 18
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23671 receive buffer: 3
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24153 receive buffer: 2
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24153 write cache: 19
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24154 receive buffer: 3
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24198 receive buffer: 2
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24198 write cache: 20
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24202 receive buffer: 3
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24305 send buffer: 0
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24305 send buffer: 1
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24909 receive buffer: 2
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24909 write cache: 21
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24910 receive buffer: 3
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The time stamp is in milliseconds since start.
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To generate a graph, use ``parse_disk_buffer_log.py``. It takes the log file as the first
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command line argument. It generates ``disk_buffer.png``.
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.. image:: disk_buffer_sample.png
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This is an example graph generated by the parse script.
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disk_access.log
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'''''''''''''''
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The disc access log has three fields. The timestamp (milliseconds since start), operation
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and offset. The offset is the absolute offset within the torrent (not within a file). This
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log is only useful when you're downloading a single torrent, otherwise the offsets will not
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be unique.
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In order to easily plot this directly in gnuplot, without parsing it, there are two lines
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associated with each read or write operation. The first one is the offset where the operation
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started, and the second one is where the operation ended.
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Example::
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15437 read 301187072
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15437 read_end 301203456
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16651 read 213385216
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16680 read_end 213647360
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25879 write 249036800
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25879 write_end 249298944
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26811 read 325582848
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26943 read_end 325844992
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36736 read 367001600
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36766 read_end 367263744
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The disk access log does not have any good visualization tool yet. There is however a gnuplot
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file, ``disk_access.gnuplot`` which assumes ``disk_access.log`` is in the current directory.
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.. image:: disk_access.png
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The density of the disk seeks tells you how hard the drive has to work.
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session stats
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-------------
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By defining ``TORRENT_STATS`` libtorrent will write a log file called ``session_stats.log`` which
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is in a format ready to be passed directly into gnuplot. The parser script ``parse_session_stats.py``
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will however parse out the field names and generate 3 different views of the data. This script
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is easy to modify to generate the particular view you're interested in.
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The first line in the log contains all the field names, separated by colon::
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second:upload rate:download rate:downloading torrents:seeding torrents:peers...
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The rest of the log is one line per second with all the fields' values.
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These are the fields:
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===================== ===============================================================
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field name description
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===================== ===============================================================
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second the time, in seconds, for this log line
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upload rate the number of bytes uploaded in the last second
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download rate the number of bytes downloaded in the last second
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downloading torrents the number of torrents that are not seeds
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seeding torrents the number of torrents that are seed
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peers the total number of connected peers
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connecting peers the total number of peers attempting to connect (half-open)
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disk block buffers the total number of disk buffer blocks that are in use
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unchoked peers the total number of unchoked peers
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num list peers the total number of known peers, but not necessarily connected
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peer allocations the total number of allocations for the peer list pool
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peer storage bytes the total number of bytes allocated for the peer list pool
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===================== ===============================================================
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This is an example of a graph that can be generated from this log:
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.. image:: session_stats_peers.png
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It shows statistics about the number of peers and peers states. How at the startup
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there are a lot of half-open connections, which tapers off as the total number of
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peers approaches the limit (50). It also shows how the total peer list slowly but steadily
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grows over time. This list is plotted against the right axis, as it has a different scale
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as the other fields.
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contributions
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=============
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If you have added instrumentation for some part of libtorrent that is not covered here, or
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if you have improved any of the parser scrips, please consider contributing it back to the
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project.
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If you have run tests and found that some algorithm or default value in libtorrent is
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suboptimal, please contribute that knowledge back as well, to allow us to improve the library.
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If you have additional suggestions on how to tune libtorrent for any specific use case,
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please let us know and we'll update this document.
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