Re: [PATCH for-next v3 0/4] fixed-buffer for uring-cmd/passthrough

Jens Axboe
2022-09-02T16:32:29Z

On 9/2/22 10:06 AM, Jens Axboe wrote:
> On 9/2/22 9:16 AM, Kanchan Joshi wrote:
>> Hi,
>>
>> Currently uring-cmd lacks the ability to leverage the pre-registered
>> buffers. This series adds the support in uring-cmd, and plumbs
>> nvme passthrough to work with it.
>>
>> Using registered-buffers showed peak-perf hike from 1.85M to 2.17M IOPS
>> in my setup.
>>
>> Without fixedbufs
>> *****************
>> # taskset -c 0 t/io_uring -b512 -d128 -c32 -s32 -p0 -F1 -B0 -O0 -n1 -u1 /dev/ng0n1
>> submitter=0, tid=5256, file=/dev/ng0n1, node=-1
>> polled=0, fixedbufs=0/0, register_files=1, buffered=1, QD=128
>> Engine=io_uring, sq_ring=128, cq_ring=128
>> IOPS=1.85M, BW=904MiB/s, IOS/call=32/31
>> IOPS=1.85M, BW=903MiB/s, IOS/call=32/32
>> IOPS=1.85M, BW=902MiB/s, IOS/call=32/32
>> ^CExiting on signal
>> Maximum IOPS=1.85M
>
> With the poll support queued up, I ran this one as well. tldr is:
>
> bdev (non pt) 122M IOPS
> irq driven 51-52M IOPS
> polled 71M IOPS
> polled+fixed 78M IOPS
>
> Looking at profiles, it looks like the bio is still being allocated
> and freed and not dipping into the alloc cache, which is using a
> substantial amount of CPU. I'll poke a bit and see what's going on...

It's using the fs_bio_set, and that doesn't have the PERCPU alloc cache
enabled. With the below, we then do:

polled+fixed 82M

I suspect the remainder is due to the lack of batching on the request
freeing side, at least some of it. Haven't really looked deeper yet.

One issue I saw - try and use passthrough polling without having any
poll queues defined and it'll stall just spinning on completions. You
need to ensure that these are processed as well - look at how the
non-passthrough io_uring poll path handles it.

diff --git a/block/bio.c b/block/bio.c
index 3d3a2678fea2..cba6b1c02eb8 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1754,7 +1754,7 @@ static int __init init_bio(void)
cpuhp_setup_state_multi(CPUHP_BIO_DEAD, "block/bio:dead", NULL,
bio_cpu_dead);

- if (bioset_init(&fs_bio_set, BIO_POOL_SIZE, 0, BIOSET_NEED_BVECS))
+ if (bioset_init(&fs_bio_set, BIO_POOL_SIZE, 0, BIOSET_NEED_BVECS | BIOSET_PERCPU_CACHE))
panic("bio: can't allocate bios\n");

if (bioset_integrity_create(&fs_bio_set, BIO_POOL_SIZE))

--
Jens Axboe