RE: [PATCH] mempool: micro optimizations

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> Sent: Wednesday, 26 February 2025 17.53
> 
> On Wed, Feb 26, 2025 at 03:59:22PM +0000, Morten Brørup wrote:
> > The comparisons lcore_id < RTE_MAX_LCORE and lcore_id != LCORE_ID_ANY
> are
> > equivalent, but the latter compiles to fewer bytes of code space.
> > Similarly for lcore_id >= RTE_MAX_LCORE and lcore_id == LCORE_ID_ANY.
> >
> > The rte_mempool_get_ops() function is also used in the fast path, so
> > RTE_VERIFY() was replaced by RTE_ASSERT().
> >
> > Compilers implicitly consider comparisons of variable == 0 likely, so
> > unlikely() was added to the check for no mempool cache (mp-
> >cache_size ==
> > 0) in the rte_mempool_default_cache() function.
> >
> > The rte_mempool_do_generic_put() function for adding objects to a
> mempool
> > was refactored as follows:
> > - The comparison for the request itself being too big, which is
> considered
> >   unlikely, was moved down and out of the code path where the cache
> has
> >   sufficient room for the added objects, which is considered the most
> >   likely code path.
> > - Added __rte_assume() about the cache length, size and threshold,
> for
> >   compiler optimization when "n" is compile time constant.
> > - Added __rte_assume() about "ret" being zero, so other functions
> using
> >   the value returned by this function can be potentially optimized by
> the
> >   compiler; especially when it merges multiple sequential code paths
> of
> >   inlined code depending on the return value being either zero or
> >   negative.
> > - The refactored source code (with comments) made the separate
> comment
> >   describing the cache flush/add algorithm superfluous, so it was
> removed.
> >
> > A few more likely()/unlikely() were added.
> >
> > A few comments were improved for readability.
> >
> > Some assertions, RTE_ASSERT(), were added. Most importantly to assert
> that
> > the return values of the mempool drivers' enqueue and dequeue
> operations
> > are API compliant, i.e. 0 (for success) or negative (for failure),
> and
> > never positive.
> >
> > Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> > ---
> >  lib/mempool/rte_mempool.h | 67 ++++++++++++++++++++++---------------
> --
> >  1 file changed, 38 insertions(+), 29 deletions(-)
> >
> Is there any measurable performance change with these modifications?

It varies.
Here are some of the good ones, tested on a VM under VMware:

mempool_autotest cache=512 cores=1
n_get_bulk=64 n_put_bulk=64 n_keep=128 constant_n=0
rate_persec=1309408130 -> 1417067889 : +8.2 %

mempool_autotest cache=512 cores=1
n_get_bulk=64 n_put_bulk=64 n_keep=128 constant_n=1
rate_persec=1479812844 -> 1573307159 : +6.3 %

mempool_autotest cache=512 cores=1
n_max_bulk=32 n_keep=128 constant_n=0
rate_persec=825183959 -> 868013386 : +5.2 %

The last result is from a new type of test where the size of every get/put varies between 1 and n_max_bulk, so the CPU's dynamic branch predictor cannot predict the request size.
I'll probably provide a separate patch for test_mempool_perf.c with this new test type, when I have finished it.