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added new section on the new disk cache optimizations to the feature page

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Arvid Norberg 2009-05-24 22:30:53 +00:00
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docs/features.rst
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@ -141,6 +141,37 @@ used to flush multiple cache blocks in a single call.
On low-memory systems, the disk cache can be disabled altogether or set to smaller
limit, to save memory.
The disk caching algorithm is configurable between 'LRU' and 'largest contiguous'.
The largest contiguous algorithm is the default and flushes the largest contiguous
block of buffers, instead of flushing all blocks belonging to the piece which was
written to least recently.
For version 0.15 a lot of work went into optimizing the cache algorithm, trying
to increase the cache hit rate and utilization. The graph to the left shows the
memory utilization in 0.14. This cache is a straight forward, fairly naive, implementation.
Every block read will also read all subsequent blocks in that piece into the cache.
Whenever we need more space, the entire oldest piece is evicted from the cache. Caching
writes always takes presedence over the read cache. Whenever a piece is fully downloaded,
it is flushed to disk.
.. image:: disk_buffer_before_optimization.png
:width: 49%
.. image:: disk_buffer.png
:width: 49%
The left graph shows the problem of evicting entire pieces at a time, and waiting until
an entire piece is downloaded until flushing it. These graphs were generated for a torrent
with fairly large pieces. This means that granularity was poor in 0.14, since it only
dealt with entire pieces. In 0.15, the granularity problem has been fixed by evicting one
block at a time from the read cache. This maximizes the read cache utilization. The write
cache is also flushed when a sufficient number of contiguous blocks have been downloaded
for a piece, which is not tied to the piece size anymore. This way the cache scales a lot
better with piece sizes.
The graph to the right shows the same download but with the new optimized disk cache
algorithm. It clearly shows an increased utilization, which means higher read hit rates
or smaller caches with maintained hit rate.
network buffers
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