Re: [dpdk-dev] ***Spam*** [RFC 2/4] mempool: unalign size when calculating required mem amount

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

On Wed, Aug 07, 2019 at 06:21:20PM +0300, Andrew Rybchenko wrote:
> On 7/19/19 4:38 PM, Olivier Matz wrote:
> > The size returned by rte_mempool_op_calc_mem_size_default() is aligned
> > to the specified page size. This means that with big pages, the returned
> > amount is more that what we really need to populate the mempool.
> > 
> > This problem is tempered by the allocation method of
> > rte_mempool_populate_default(): in some conditions (when
> > try_iova_contig_mempool=true), it first tries to allocate all objs
> > memory in an iova contiguous area, without the alignment constraint. If
> > it fails, it fallbacks to the big aligned allocation, that can also
> > fallback into several smaller allocations.
> > 
> > This commit changes rte_mempool_op_calc_mem_size_default() to return the
> > unaligned amount of memory (the alignment constraint is still returned
> > via the *align argument),
> 
> It does not as far as I can see. There is no changes in
> lib/librte_mempool/rte_mempool_ops_default.c

True, sorry. This is fixed in the new version.

> > and removes the optimistic contiguous
> > allocation done when try_iova_contig_mempool=true.
> 
> Unfortunately I don't understand why these 2 changes are combined
> into to one patch. The first looks good to me, the second is
> questionable. It can impact performance.

I splited this change into 2 patches, that's indeed clearer.

However I didn't get what kind do performance impact do you expect?

> > This will make the amount of allocated memory more predictible: it will
> > be more than the optimistic contiguous allocation, but less than the big
> > aligned allocation.
> > 
> > This opens the door for the next commits that will try to prevent objets
> > from being located across pages.
> > 
> > Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> > ---
> >   lib/librte_mempool/rte_mempool.c     | 44 ++++--------------------------------
> >   lib/librte_mempool/rte_mempool.h     |  2 +-
> >   lib/librte_mempool/rte_mempool_ops.c |  4 +++-
> >   3 files changed, 9 insertions(+), 41 deletions(-)
> > 
> > diff --git a/lib/librte_mempool/rte_mempool.c b/lib/librte_mempool/rte_mempool.c
> > index 0f29e8712..335032dc8 100644
> > --- a/lib/librte_mempool/rte_mempool.c
> > +++ b/lib/librte_mempool/rte_mempool.c
> > @@ -430,7 +430,6 @@ rte_mempool_populate_default(struct rte_mempool *mp)
> >   	unsigned mz_id, n;
> >   	int ret;
> >   	bool need_iova_contig_obj;
> > -	bool try_iova_contig_mempool;
> >   	bool alloc_in_ext_mem;
> >   	ret = mempool_ops_alloc_once(mp);
> > @@ -477,18 +476,10 @@ rte_mempool_populate_default(struct rte_mempool *mp)
> >   	 * wasting some space this way, but it's much nicer than looping around
> >   	 * trying to reserve each and every page size.
> >   	 *
> > -	 * However, since size calculation will produce page-aligned sizes, it
> > -	 * makes sense to first try and see if we can reserve the entire memzone
> > -	 * in one contiguous chunk as well (otherwise we might end up wasting a
> > -	 * 1G page on a 10MB memzone). If we fail to get enough contiguous
> > -	 * memory, then we'll go and reserve space page-by-page.
> > -	 *
> >   	 * We also have to take into account the fact that memory that we're
> >   	 * going to allocate from can belong to an externally allocated memory
> >   	 * area, in which case the assumption of IOVA as VA mode being
> > -	 * synonymous with IOVA contiguousness will not hold. We should also try
> > -	 * to go for contiguous memory even if we're in no-huge mode, because
> > -	 * external memory may in fact be IOVA-contiguous.
> > +	 * synonymous with IOVA contiguousness will not hold.
> >   	 */
> >   	/* check if we can retrieve a valid socket ID */
> > @@ -497,7 +488,6 @@ rte_mempool_populate_default(struct rte_mempool *mp)
> >   		return -EINVAL;
> >   	alloc_in_ext_mem = (ret == 1);
> >   	need_iova_contig_obj = !(mp->flags & MEMPOOL_F_NO_IOVA_CONTIG);
> > -	try_iova_contig_mempool = false;
> >   	if (!need_iova_contig_obj) {
> >   		pg_sz = 0;
> > @@ -506,7 +496,6 @@ rte_mempool_populate_default(struct rte_mempool *mp)
> >   		pg_sz = 0;
> >   		pg_shift = 0;
> >   	} else if (rte_eal_has_hugepages() || alloc_in_ext_mem) {
> > -		try_iova_contig_mempool = true;
> >   		pg_sz = get_min_page_size(mp->socket_id);
> >   		pg_shift = rte_bsf32(pg_sz);
> >   	} else {
> > @@ -518,12 +507,8 @@ rte_mempool_populate_default(struct rte_mempool *mp)
> >   		size_t min_chunk_size;
> >   		unsigned int flags;
> > -		if (try_iova_contig_mempool || pg_sz == 0)
> > -			mem_size = rte_mempool_ops_calc_mem_size(mp, n,
> > -					0, &min_chunk_size, &align);
> > -		else
> > -			mem_size = rte_mempool_ops_calc_mem_size(mp, n,
> > -					pg_shift, &min_chunk_size, &align);
> > +		mem_size = rte_mempool_ops_calc_mem_size(
> > +			mp, n, pg_shift, &min_chunk_size, &align);
> >   		if (mem_size < 0) {
> >   			ret = mem_size;
> > @@ -542,31 +527,12 @@ rte_mempool_populate_default(struct rte_mempool *mp)
> >   		/* if we're trying to reserve contiguous memory, add appropriate
> >   		 * memzone flag.
> >   		 */
> > -		if (try_iova_contig_mempool)
> > +		if (min_chunk_size == (size_t)mem_size)
> >   			flags |= RTE_MEMZONE_IOVA_CONTIG;
> >   		mz = rte_memzone_reserve_aligned(mz_name, mem_size,
> >   				mp->socket_id, flags, align);
> > -		/* if we were trying to allocate contiguous memory, failed and
> > -		 * minimum required contiguous chunk fits minimum page, adjust
> > -		 * memzone size to the page size, and try again.
> > -		 */
> > -		if (mz == NULL && try_iova_contig_mempool &&
> > -				min_chunk_size <= pg_sz) {
> > -			try_iova_contig_mempool = false;
> > -			flags &= ~RTE_MEMZONE_IOVA_CONTIG;
> > -
> > -			mem_size = rte_mempool_ops_calc_mem_size(mp, n,
> > -					pg_shift, &min_chunk_size, &align);
> > -			if (mem_size < 0) {
> > -				ret = mem_size;
> > -				goto fail;
> > -			}
> > -
> > -			mz = rte_memzone_reserve_aligned(mz_name, mem_size,
> > -				mp->socket_id, flags, align);
> > -		}
> >   		/* don't try reserving with 0 size if we were asked to reserve
> >   		 * IOVA-contiguous memory.
> >   		 */
> > @@ -594,7 +560,7 @@ rte_mempool_populate_default(struct rte_mempool *mp)
> >   		else
> >   			iova = RTE_BAD_IOVA;
> > -		if (try_iova_contig_mempool || pg_sz == 0)
> > +		if (pg_sz == 0)
> >   			ret = rte_mempool_populate_iova(mp, mz->addr,
> >   				iova, mz->len,
> >   				rte_mempool_memchunk_mz_free,
> > diff --git a/lib/librte_mempool/rte_mempool.h b/lib/librte_mempool/rte_mempool.h
> > index 8053f7a04..7bc10e699 100644
> > --- a/lib/librte_mempool/rte_mempool.h
> > +++ b/lib/librte_mempool/rte_mempool.h
> > @@ -458,7 +458,7 @@ typedef unsigned (*rte_mempool_get_count)(const struct rte_mempool *mp);
> >    * @param[out] align
> >    *   Location for required memory chunk alignment.
> >    * @return
> > - *   Required memory size aligned at page boundary.
> > + *   Required memory size.
> >    */
> >   typedef ssize_t (*rte_mempool_calc_mem_size_t)(const struct rte_mempool *mp,
> >   		uint32_t obj_num,  uint32_t pg_shift,
> > diff --git a/lib/librte_mempool/rte_mempool_ops.c b/lib/librte_mempool/rte_mempool_ops.c
> > index e02eb702c..22c5251eb 100644
> > --- a/lib/librte_mempool/rte_mempool_ops.c
> > +++ b/lib/librte_mempool/rte_mempool_ops.c
> > @@ -100,7 +100,9 @@ rte_mempool_ops_get_count(const struct rte_mempool *mp)
> >   	return ops->get_count(mp);
> >   }
> > -/* wrapper to notify new memory area to external mempool */
> > +/* wrapper to calculate the memory size required to store given number
> > + * of objects
> > + */
> >   ssize_t
> >   rte_mempool_ops_calc_mem_size(const struct rte_mempool *mp,
> >   				uint32_t obj_num, uint32_t pg_shift,
>