Re: [dpdk-dev] [PATCH] mempool: optimize copy in cache get

[Olivier Matz <olivier.matz@6wind.com>]

Hi,

On Tue, May 21, 2019 at 12:34:55PM +0300, Andrew Rybchenko wrote:
> On 5/21/19 12:03 PM, Xiao Wang wrote:
> > Use rte_memcpy to improve the pointer array copy. This optimization method
> > has already been applied to __mempool_generic_put() [1], this patch applies
> > it to __mempool_generic_get(). Slight performance gain can be observed in
> > testpmd txonly test.
> > 
> > [1] 863bfb47449 ("mempool: optimize copy in cache")
> > 
> > Signed-off-by: Xiao Wang <xiao.w.wang@intel.com>
> > ---
> >   lib/librte_mempool/rte_mempool.h | 7 +------
> >   1 file changed, 1 insertion(+), 6 deletions(-)
> > 
> > diff --git a/lib/librte_mempool/rte_mempool.h b/lib/librte_mempool/rte_mempool.h
> > index 8053f7a04..975da8d22 100644
> > --- a/lib/librte_mempool/rte_mempool.h
> > +++ b/lib/librte_mempool/rte_mempool.h
> > @@ -1344,15 +1344,11 @@ __mempool_generic_get(struct rte_mempool *mp, void **obj_table,
> >   		      unsigned int n, struct rte_mempool_cache *cache)
> >   {
> >   	int ret;
> > -	uint32_t index, len;
> > -	void **cache_objs;
> >   	/* No cache provided or cannot be satisfied from cache */
> >   	if (unlikely(cache == NULL || n >= cache->size))
> >   		goto ring_dequeue;
> > -	cache_objs = cache->objs;
> > -
> >   	/* Can this be satisfied from the cache? */
> >   	if (cache->len < n) {
> >   		/* No. Backfill the cache first, and then fill from it */
> > @@ -1375,8 +1371,7 @@ __mempool_generic_get(struct rte_mempool *mp, void **obj_table,
> >   	}
> >   	/* Now fill in the response ... */
> > -	for (index = 0, len = cache->len - 1; index < n; ++index, len--, obj_table++)
> > -		*obj_table = cache_objs[len];
> > +	rte_memcpy(obj_table, &cache->objs[cache->len - n], sizeof(void *) * n);
> >   	cache->len -= n;
> 
> I think the idea of the loop above is to get objects in reverse order to
> order
> to reuse cache top objects (put last) first. It should improve cache hit
> etc.
> So, performance effect of the patch could be very different on various CPUs
> (with different cache sizes) and various work-loads.
> 
> So, I doubt that it is a step in right direction.

For reference, this was already discussed 3 years ago:

https://mails.dpdk.org/archives/dev/2016-May/039873.html
https://mails.dpdk.org/archives/dev/2016-June/040029.html

I'm still not convinced that reversing object addresses (as it's done
today) is really important. But Andrew is probably right, the impact of
this kind of patch probably varies depending on many factors. More
performance numbers on real-life use-cases would help to decide what to
do.

Regards,
Olivier