Re: [PATCH v6] mempool: improve cache behaviour and performance

[Thomas Monjalon <thomas@monjalon.net>]

26/05/2026 18:00, Morten Brørup:
> > From: Morten Brørup [mailto:mb@smartsharesystems.com]
> > Sent: Tuesday, 26 May 2026 16.00
> > 
> > This patch refactors the mempool cache to eliminate some unexpected
> > behaviour and reduce the mempool cache miss rate.
> > 
> > 1.
> > The actual cache size was 1.5 times the cache size specified at run-
> > time
> > mempool creation.
> > This was obviously not expected by application developers.
> > 
> > 2.
> > In get operations, the check for when to use the cache as bounce buffer
> > did not respect the run-time configured cache size,
> > but compared to the build time maximum possible cache size
> > (RTE_MEMPOOL_CACHE_MAX_SIZE, default 512).
> > E.g. with a configured cache size of 32 objects, getting 256 objects
> > would first fetch 32 + 256 = 288 objects into the cache,
> > and then move the 256 objects from the cache to the destination memory,
> > instead of fetching the 256 objects directly to the destination memory.
> > This had a performance cost.
> > However, this is unlikely to occur in real applications, so it is not
> > important in itself.
> > 
> > 3.
> > When putting objects into a mempool, and the mempool cache did not have
> > free space for so many objects,
> > the cache was flushed completely, and the new objects were then put
> > into
> > the cache.
> > I.e. the cache drain level was zero.
> > This (complete cache flush) meant that a subsequent get operation (with
> > the same number of objects) completely emptied the cache,
> > so another subsequent get operation required replenishing the cache.
> > 
> > Similarly,
> > When getting objects from a mempool, and the mempool cache did not hold
> > so
> > many objects,
> > the cache was replenished to cache->size + remaining objects,
> > and then (the remaining part of) the requested objects were fetched via
> > the cache,
> > which left the cache filled (to cache->size) at completion.
> > I.e. the cache refill level was cache->size (plus some, depending on
> > request size).
> > 
> > (1) was improved by generally comparing to cache->size instead of
> > cache->flushthresh, when considering the capacity of the cache.
> > The cache->flushthresh field is kept for API/ABI compatibility
> > purposes,
> > and initialized to cache->size instead of cache->size * 1.5.
> > 
> > (2) was improved by generally comparing to cache->size / 2 instead of
> > RTE_MEMPOOL_CACHE_MAX_SIZE, when checking the bounce buffer limit.
> > 
> > (3) was improved by flushing and replenishing the cache by half its
> > size,
> > so a flush/refill can be followed randomly by get or put requests.
> > This also reduced the number of objects in each flush/refill operation.
> > 
> > As a consequence of these changes, the size of the array holding the
> > objects in the cache (cache->objs[]) no longer needs to be
> > 2 * RTE_MEMPOOL_CACHE_MAX_SIZE, and can be reduced to
> > RTE_MEMPOOL_CACHE_MAX_SIZE at an API/ABI breaking release.

I'm not sure why waiting?


> > Performance data:
> > With a real WAN Optimization application, where the number of allocated
> > packets varies (as they are held in e.g. shaper queues), the mempool
> > cache miss rate dropped from ca. 1/20 objects to ca. 1/48 objects.
> > This was deployed in production at an ISP, and using an effective cache
> > size of 384 objects.
> > 
> > Bugzilla ID: 1027
> > Fixes: ea5dd2744b90 ("mempool: cache optimisations")
> > Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> 
> Forgot carrying an Ack over from v5:
> Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
> 
> > ---
> > Depends-on: patch-163181 ("net/intel: do not bypass mbuf lib for mbuf
> > fast-free")
> 
> This dependency seems to cause CI apply failures.
> The dependency is based on an older snapshot of main,
> and this patch is based on a new snapshot of main.

The dependency should be resolved now.
Please could you send a v7?