Re: [PATCH 0/5] aio-posix: towards an O(1) event loop

[Stefan Hajnoczi]

On Fri, Feb 14, 2020 at 05:17:07PM +0000, Stefan Hajnoczi wrote:
> This patch series makes AioHandler deletion and dispatch O(1) with respect to
> the total number of registered handlers.  The event loop has scalability
> problems when many AioHandlers are registered because it is O(n).  Linux
> epoll(7) is used to avoid scanning over all pollfds but parts of the code 
> still
> scan all AioHandlers.
> 
> This series reduces QEMU CPU utilization and therefore increases IOPS,
> especially for guests that have many devices.  It was tested with 32 vCPUs, 2
> virtio-blk,num-queues=1,iothread=iothread1, and 99
> virtio-blk,num-queues=32,iothread=iothread1 devices.  Using an IOThread is
> necessary because this series does not improve the glib main loop, a non-goal
> since the glib API is inherently O(n).
> 
> AioContext polling remains O(n) and will be addressed in a separate patch
> series.  This patch series increases IOPS from 260k to 300k when AioContext
> polling is commented out
> (rw=randread,bs=4k,iodepth=32,ioengine=libaio,direct=1).
> 
> Stefan Hajnoczi (5):
>   aio-posix: fix use after leaving scope in aio_poll()
>   aio-posix: don't pass ns timeout to epoll_wait()
>   qemu/queue.h: add QLIST_SAFE_REMOVE()
>   aio-posix: make AioHandler deletion O(1)
>   aio-posix: make AioHandler dispatch O(1) with epoll
> 
>  block.c              |   5 +-
>  chardev/spice.c      |   4 +-
>  include/block/aio.h  |   6 +-
>  include/qemu/queue.h |  17 +++++
>  util/aio-posix.c     | 172 +++++++++++++++++++++++++++++--------------
>  5 files changed, 141 insertions(+), 63 deletions(-)
> 
> -- 
> 2.24.1
> 

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan

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