Hi,

I’ve tracked down a regression in SPDK to DPDK commit a2833ecc5 (“mempool: fix get objects from mempool with cache”).

Here’s an example that demonstrates the problem:

Allocate mempool with 2048 buffers and cache size 256.
Core 0 allocates 512 buffers.  Mempool pulls 512 + 256 buffers from backing ring, returns 512 of them to caller, puts the other 256 in core 0 cache.  Backing ring now has 1280 buffers.
Core 1 allocates 512 buffers.  Mempool pulls 512 + 256 buffers from backing ring, returns 512 of them to caller, puts the other 256 in core 1 cache.  Backing ring now has 512 buffers.
Core 2 allocates 512 buffers.  Mempool pulls remaining 512 buffers from backing ring and returns all of them to caller.  Backing ring now has 0 buffers.
Core 3 tries to allocate 512 buffers and it fails.

In the SPDK case, we don’t really need or use the mempool cache in this case, so changing the cache size to 0 fixes the problem and is what we’re going to move forward with.

But the behavior did cause a regression so I thought I’d mention it here.  If you have a mempool with 2048 objects, shouldn’t 4 cores each be able to do a 512 buffer bulk get, regardless of the configured cache size?

Regards,

Jim Harris