Re: [dpdk-dev] [PATCH] mempool: fix slow allocation of large mempools

[Andrew Rybchenko <arybchenko@solarflare.com>]

On 1/9/20 4:27 PM, Olivier Matz wrote:
> When allocating a mempool which is larger than the largest
> available area, it can take a lot of time:
> 
> a- the mempool calculate the required memory size, and tries
>    to allocate it, it fails
> b- then it tries to allocate the largest available area (this
>    does not request new huge pages)
> c- add this zone to the mempool, this triggers the allocation
>    of a mem hdr, which request a new huge page
> d- back to a- until mempool is populated or until there is no
>    more memory
> 
> This can take a lot of time to finally fail (several minutes): in step
> a- it takes all available hugepages on the system, then release them
> after it fails.
> 
> The problem appeared with commit eba11e364614 ("mempool: reduce wasted
> space on populate"), because smaller chunks are now allowed. Previously,
> it had to be at least one page size, which is not the case in step b-.
> 
> To fix this, implement our own way to allocate the largest available
> area instead of using the feature from memzone: if an allocation fails,
> try to divide the size by 2 and retry. When the requested size falls
> below min_chunk_size, stop and return an error.
> 
> Fixes: eba11e364614 ("mempool: reduce wasted space on populate")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>

LGTM except already mentioned bug with missing mz == NULL to retry loop.
Plus one minor question below.

> ---
>  lib/librte_mempool/rte_mempool.c | 29 ++++++++++++-----------------
>  1 file changed, 12 insertions(+), 17 deletions(-)
> 
> diff --git a/lib/librte_mempool/rte_mempool.c b/lib/librte_mempool/rte_mempool.c
> index bda361ce6..03c8d984c 100644
> --- a/lib/librte_mempool/rte_mempool.c
> +++ b/lib/librte_mempool/rte_mempool.c
> @@ -481,6 +481,7 @@ rte_mempool_populate_default(struct rte_mempool *mp)
>  	unsigned mz_id, n;
>  	int ret;
>  	bool need_iova_contig_obj;
> +	size_t max_alloc_size = SIZE_MAX;
>  
>  	ret = mempool_ops_alloc_once(mp);
>  	if (ret != 0)
> @@ -560,30 +561,24 @@ rte_mempool_populate_default(struct rte_mempool *mp)
>  		if (min_chunk_size == (size_t)mem_size)
>  			mz_flags |= RTE_MEMZONE_IOVA_CONTIG;
>  
> -		mz = rte_memzone_reserve_aligned(mz_name, mem_size,
> +		/* Allocate a memzone, retrying with a smaller area on ENOMEM */
> +		do {
> +			mz = rte_memzone_reserve_aligned(mz_name,
> +				RTE_MIN((size_t)mem_size, max_alloc_size),
>  				mp->socket_id, mz_flags, align);
>  
> -		/* don't try reserving with 0 size if we were asked to reserve
> -		 * IOVA-contiguous memory.
> -		 */
> -		if (min_chunk_size < (size_t)mem_size && mz == NULL) {
> -			/* not enough memory, retry with the biggest zone we
> -			 * have
> -			 */
> -			mz = rte_memzone_reserve_aligned(mz_name, 0,
> -					mp->socket_id, mz_flags, align);
> -		}
> +			if (mz == NULL && rte_errno != ENOMEM)
> +				break;
> +
> +			max_alloc_size = RTE_MIN(max_alloc_size,
> +						(size_t)mem_size) / 2;

Does it make sense to make max_alloc_size multiple of
min_chunk_size here? I think it could help to waste less
memory space.

> +		} while (max_alloc_size >= min_chunk_size);
> +
>  		if (mz == NULL) {
>  			ret = -rte_errno;
>  			goto fail;
>  		}
>  
> -		if (mz->len < min_chunk_size) {
> -			rte_memzone_free(mz);
> -			ret = -ENOMEM;
> -			goto fail;
> -		}
> -
>  		if (need_iova_contig_obj)
>  			iova = mz->iova;
>  		else
>