I posted v3 on August 7. Nobody acked or merged the patches, and then
I got too busy with other stuff to repost until now.
The only change since v3:
*) Dropped patch #10 (the mpt3sas patch) since the mpt3sas maintainers
didn't show any interest.
I believe these patches are ready for merging.
---
drivers/scsi/mpt3sas is running into a scalability problem with the
kernel's DMA pool implementation. With a LSI/Broadcom SAS 9300-8i
12Gb/s HBA and max_sgl_entries=256, during modprobe, mpt3sas does the
equivalent of:
chain_dma_pool = dma_pool_create(size = 128);
for (i = 0; i < 373959; i++)
{
dma_addr[i] = dma_pool_alloc(chain_dma_pool);
}
And at rmmod, system shutdown, or system reboot, mpt3sas does the
equivalent of:
for (i = 0; i < 373959; i++)
{
dma_pool_free(chain_dma_pool, dma_addr[i]);
}
dma_pool_destroy(chain_dma_pool);
With this usage, both dma_pool_alloc() and dma_pool_free() exhibit
O(n^2) complexity, although dma_pool_free() is much worse due to
implementation details. On my system, the dma_pool_free() loop above
takes about 9 seconds to run. Note that the problem was even worse
before commit 74522a92bbf0 ("scsi: mpt3sas: Optimize I/O memory
consumption in driver."), where the dma_pool_free() loop could take ~30
seconds.
mpt3sas also has some other DMA pools, but chain_dma_pool is the only
one with so many allocations:
cat /sys/devices/pci0000:80/0000:80:07.0/0000:85:00.0/pools
(manually cleaned up column alignment)
poolinfo - 0.1
reply_post_free_array pool 1 21 192 1
reply_free pool 1 1 41728 1
reply pool 1 1 1335296 1
sense pool 1 1 970272 1
chain pool 373959 386048 128 12064
reply_post_free pool 12 12 166528 12
The patches in this series improve the scalability of the DMA pool
implementation, which significantly reduces the running time of the
DMA alloc/free loops. With the patches applied, "modprobe mpt3sas",
"rmmod mpt3sas", and system shutdown/reboot with mpt3sas loaded are
significantly faster. Here are some benchmarks (of DMA alloc/free
only, not the entire modprobe/rmmod):
dma_pool_create() + dma_pool_alloc() loop, size = 128, count = 373959
original: 350 ms ( 1x)
dmapool patches: 17 ms (21x)
dma_pool_free() loop + dma_pool_destroy(), size = 128, count = 373959
original: 8901 ms ( 1x)
dmapool patches: 15 ms ( 618x)
