[v2] mempool: fix put objects to mempool with cache

  This patch optimizes the rte_mempool_do_generic_put() caching algorithm,
and fixes a bug in it.

The existing algorithm was:
 1. Add the objects to the cache
 2. Anything greater than the cache size (if it crosses the cache flush
    threshold) is flushed to the ring.

Please note that the description in the source code said that it kept
"cache min value" objects after flushing, but the function actually kept
"size" objects, which is reflected in the above description.

Now, the algorithm is:
 1. If the objects cannot be added to the cache without crossing the
    flush threshold, flush the cache to the ring.
 2. Add the objects to the cache.

This patch changes these details:

1. Bug: The cache was still full after flushing.
In the opposite direction, i.e. when getting objects from the cache, the
cache is refilled to full level when it crosses the low watermark (which
happens to be zero).
Similarly, the cache should be flushed to empty level when it crosses
the high watermark (which happens to be 1.5 x the size of the cache).
The existing flushing behaviour was suboptimal for real applications,
because crossing the low or high watermark typically happens when the
application is in a state where the number of put/get events are out of
balance, e.g. when absorbing a burst of packets into a QoS queue
(getting more mbufs from the mempool), or when a burst of packets is
trickling out from the QoS queue (putting the mbufs back into the
mempool).
NB: When the application is in a state where put/get events are in
balance, the cache should remain within its low and high watermarks, and
the algorithms for refilling/flushing the cache should not come into
play.
Now, the mempool cache is completely flushed when crossing the flush
threshold, so only the newly put (hot) objects remain in the mempool
cache afterwards.

2. Minor bug: The flush threshold comparison has been corrected; it must
be "len > flushthresh", not "len >= flushthresh".
Reasoning: Consider a flush multiplier of 1 instead of 1.5; the cache
would be flushed already when reaching size elements, not when exceeding
size elements.
Now, flushing is triggered when the flush threshold is exceeded, not
when reached.

3. Optimization: The most recent (hot) objects are flushed, leaving the
oldest (cold) objects in the mempool cache.
This is bad for CPUs with a small L1 cache, because when they get
objects from the mempool after the mempool cache has been flushed, they
get cold objects instead of hot objects.
Now, the existing (cold) objects in the mempool cache are flushed before
the new (hot) objects are added the to the mempool cache.

4. Optimization: Using the x86 variant of rte_memcpy() is inefficient
here, where n is relatively small and unknown at compile time.
Now, it has been replaced by an alternative copying method, optimized
for the fact that most Ethernet PMDs operate in bursts of 4 or 8 mbufs
or multiples thereof.

v2 changes:

- Not adding the new objects to the mempool cache before flushing it
also allows the memory allocated for the mempool cache to be reduced
from 3 x to 2 x RTE_MEMPOOL_CACHE_MAX_SIZE.
However, such this change would break the ABI, so it was removed in v2.

- The mempool cache should be cache line aligned for the benefit of the
copying method, which on some CPU architectures performs worse on data
crossing a cache boundary.
However, such this change would break the ABI, so it was removed in v2;
and yet another alternative copying method replaced the rte_memcpy().

Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
---
 lib/mempool/rte_mempool.h | 54 +++++++++++++++++++++++++++++----------
 1 file changed, 40 insertions(+), 14 deletions(-)