Re: [PATCH] virtio-blk: iothread-vq-mapping coroutine pool sizing

[Kevin Wolf]

Am 11.03.2024 um 21:14 hat Stefan Hajnoczi geschrieben:
> It is possible to hit the sysctl vm.max_map_count limit when the
> coroutine pool size becomes large. Each coroutine requires two mappings
> (one for the stack and one for the guard page). QEMU can crash with
> "failed to set up stack guard page" or "failed to allocate memory for
> stack" when this happens.
> 
> Coroutine pool sizing is simple when there is only one AioContext: sum
> up all I/O requests across all virtqueues.
> 
> When the iothread-vq-mapping option is used we should calculate tighter
> bounds: take the maximum number of the number of I/O requests across all
> virtqueues. This number is lower than simply summing all virtqueues when
> only a subset of the virtqueues is handled by each AioContext.

The reasoning is that each thread has its own coroutine pool for which
the pool size applies individually, and it doesn't need to have space
for coroutines running in a different thread, right? I'd like to have
this recorded in the commit message.

Of course, this also makes me wonder if a global coroutine pool size
really makes sense or if it should be per thread. One thread could be
serving only one queue (maybe the main thread with a CD-ROM device) and
another thread 32 queues (the iothread with the interesting disks).
There is no reason for the first thread to have a coroutine pool as big
as the second one.

But before we make the size thread-local, maybe having thread-local
pools wasn't right to begin with because multiple threads can run main
context code and they should therefore share the same coroutine pool (we
already had the problem earlier that coroutines start on the vcpu thread
and terminate on the main thread and this plays havoc with coroutine
pools).

Maybe per-AioContext pools with per-AioContext sizes would make more
sense?

> This is not a solution to hitting vm.max_map_count, but it helps. A
> guest with 64 vCPUs (hence 64 virtqueues) across 4 IOThreads with one
> iothread-vq-mapping virtio-blk device and a root disk without goes from
> pool_max_size 16,448 to 10,304.
> 
> Reported-by: Sanjay Rao <srao@redhat.com>
> Reported-by: Boaz Ben Shabat <bbenshab@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Either way, this should already strictly improve the situation, so I'm
happy to apply this change for now.

Kevin