Re: [PATCH] coroutine: resize pool periodically instead of limiting size

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 9/1/21 6:09 PM, Stefan Hajnoczi wrote:
> It was reported that enabling SafeStack reduces IOPS significantly
> (>25%) with the following fio benchmark on virtio-blk using a NVMe host
> block device:
> 
>   # fio --rw=randrw --bs=4k --iodepth=64 --runtime=1m --direct=1 \
> 	--filename=/dev/vdb --name=job1 --ioengine=libaio --thread \
> 	--group_reporting --numjobs=16 --time_based \
>         --output=/tmp/fio_result
> 
> Serge Guelton and I found that SafeStack is not really at fault, it just
> increases the cost of coroutine creation. This fio workload exhausts the
> coroutine pool and coroutine creation becomes a bottleneck. Previous
> work by Honghao Wang also pointed to excessive coroutine creation.
> 
> Creating new coroutines is expensive due to allocating new stacks with
> mmap(2) and mprotect(2). Currently there are thread-local and global
> pools that recycle old Coroutine objects and their stacks but the
> hardcoded size limit of 64 for thread-local pools and 128 for the global
> pool is insufficient for the fio benchmark shown above.
> 
> This patch changes the coroutine pool algorithm to a simple thread-local
> pool without a size limit. Threads periodically shrink the pool down to
> a size sufficient for the maximum observed number of coroutines.
> 
> This is a very simple algorithm. Fancier things could be done like
> keeping a minimum number of coroutines around to avoid latency when a
> new coroutine is created after a long period of inactivity. Another
> thought is to stop the timer when the pool size is zero for power saving
> on threads that aren't using coroutines. However, I'd rather not add
> bells and whistles unless they are really necessary.
> 
> The global pool is removed by this patch. It can help to hide the fact
> that local pools are easily exhausted, but it's doesn't fix the root
> cause. I don't think there is a need for a global pool because QEMU's
> threads are long-lived, so let's keep things simple.
> 
> Performance of the above fio benchmark is as follows:
> 
>       Before   After
> IOPS     60k     97k
> 
> Memory usage varies over time as needed by the workload:
> 
>             VSZ (KB)             RSS (KB)
> Before fio  4705248              843128
> During fio  5747668 (+ ~100 MB)  849280
> After fio   4694996 (- ~100 MB)  845184
> 
> This confirms that coroutines are indeed being freed when no longer
> needed.
> 
> Thanks to Serge Guelton for working on identifying the bottleneck with
> me!
> 
> Reported-by: Tingting Mao <timao@redhat.com>
> Cc: Serge Guelton <sguelton@redhat.com>
> Cc: Honghao Wang <wanghonghao@bytedance.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Daniele Buono <dbuono@linux.vnet.ibm.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  include/qemu/coroutine-pool-timer.h | 36 +++++++++++++++++
>  include/qemu/coroutine.h            |  7 ++++
>  iothread.c                          |  6 +++
>  util/coroutine-pool-timer.c         | 35 ++++++++++++++++
>  util/main-loop.c                    |  5 +++
>  util/qemu-coroutine.c               | 62 ++++++++++++++---------------
>  util/meson.build                    |  1 +
>  7 files changed, 119 insertions(+), 33 deletions(-)
>  create mode 100644 include/qemu/coroutine-pool-timer.h
>  create mode 100644 util/coroutine-pool-timer.c

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>