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virtio: use defer_call() in virtio_irqfd_notify()

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virtio-blk and virtio-scsi invoke virtio_irqfd_notify() to send Used
Buffer Notifications from an IOThread. This involves an eventfd
write(2) syscall. Calling this repeatedly when completing multiple I/O
requests in a row is wasteful.

Use the defer_call() API to batch together virtio_irqfd_notify() calls
made during thread pool (aio=threads), Linux AIO (aio=native), and
io_uring (aio=io_uring) completion processing.

Behavior is unchanged for emulated devices that do not use
defer_call_begin()/defer_call_end() since defer_call() immediately
invokes the callback when called outside a
defer_call_begin()/defer_call_end() region.

fio rw=randread bs=4k iodepth=64 numjobs=8 IOPS increases by ~9% with a
single IOThread and 8 vCPUs. iodepth=1 decreases by ~1% but this could
be noise. Detailed performance data and configuration specifics are
available here:
https://gitlab.com/stefanha/virt-playbooks/-/tree/blk_io_plug-irqfd

This duplicates the BH that virtio-blk uses for batching. The next
commit will remove it.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230913200045.1024233-4-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2023-09-13 16:00:44 -04:00 • committed by Kevin Wolf
parent 433fcea40c
commit 84d61e5f36
5 changed files with 28 additions and 1 deletions
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6
block/io_uring.c
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@ -125,6 +125,9 @@ static void luring_process_completions(LuringState *s)
{
struct io_uring_cqe *cqes;
int total_bytes;
defer_call_begin();
/*
* Request completion callbacks can run the nested event loop.
* Schedule ourselves so the nested event loop will "see" remaining
@ -217,7 +220,10 @@ end:
aio_co_wake(luringcb->co);
}
}
qemu_bh_cancel(s->completion_bh);
defer_call_end();
}
static int ioq_submit(LuringState *s)