[Qemu-devel] [PATCH 0/6] qcow2 L2/refcount cache improvements

[Alberto Garcia]

Here are some improvements to the qcow2 L2/refcount cache code.

The first one is that all cache tables are now allocated using a
single memory block, as we discussed last week.

Apart from a more efficient use of memory, this allows some additional
optimizations so I took the chance to make other changes.

- qcow2_cache_put() and qcow2_cache_entry_mark_dirty() are now O(1)
- The eviction algorithm is now LRU. The previous one only works well
  with very small cache sizes.
- qcow2_cache_find_entry_to_replace() is no longer necessary.
- Lookups are faster now.

In my tests with a preallocated 128MB L2 cache in an empty drive the
new code is ~13% faster than the previous one (~43% if compiled
without optimizations). This is a best-case scenario, if the cache is
smaller or the drive is full of data the improvements are not so
visible, but I believe the code is simpler now so I hope you find the
changes worthwhile.

Regards,

Berto

Alberto Garcia (6):
  qcow2: use one single memory block for the L2/refcount cache tables
  qcow2: simplify qcow2_cache_put() and qcow2_cache_entry_mark_dirty()
  qcow2: use an LRU algorithm to replace entries from the L2 cache
  qcow2: remove qcow2_cache_find_entry_to_replace()
  qcow2: use a hash to look for entries in the L2 cache
  qcow2: style fixes in qcow2-cache.c

 block/qcow2-cache.c | 149 +++++++++++++++++++++-------------------------------
 1 file changed, 61 insertions(+), 88 deletions(-)

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2.1.4