Re: [Qemu-devel] [PATCH] iothread: enable AioContext polling by default

[Paolo Bonzini <pbonzini@redhat.com>]

On 26/01/2017 18:01, Stefan Hajnoczi wrote:
> IOThread AioContexts are likely to consist only of event sources like
> virtqueue ioeventfds and LinuxAIO completion eventfds that are pollable
> from userspace (without system calls).
> 
> We recently merged the AioContext polling feature but didn't enable it
> by default yet.  I have gone back over the performance data on the
> mailing list and picked a default polling value that gave good results.
> 
> Let's enable AioContext polling by default so users don't have another
> switch they need to set manually.  If performance regressions are found
> we can still disable this for the QEMU 2.9 release.
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> Cc: Karl Rister <krister@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  iothread.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/iothread.c b/iothread.c
> index 7bedde8..257b01d 100644
> --- a/iothread.c
> +++ b/iothread.c
> @@ -30,6 +30,12 @@ typedef ObjectClass IOThreadClass;
>  #define IOTHREAD_CLASS(klass) \
>     OBJECT_CLASS_CHECK(IOThreadClass, klass, TYPE_IOTHREAD)
>  
> +/* Benchmark results from 2016 on NVMe SSD drives show max polling times around
> + * 16-32 microseconds yield IOPS improvements for both iodepth=1 and iodepth=32
> + * workloads.
> + */
> +#define IOTHREAD_POLL_MAX_NS_DEFAULT 32768ULL
> +
>  static __thread IOThread *my_iothread;
>  
>  AioContext *qemu_get_current_aio_context(void)
> @@ -71,6 +77,13 @@ static int iothread_stop(Object *object, void *opaque)
>      return 0;
>  }
>  
> +static void iothread_instance_init(Object *obj)
> +{
> +    IOThread *iothread = IOTHREAD(obj);
> +
> +    iothread->poll_max_ns = IOTHREAD_POLL_MAX_NS_DEFAULT;
> +}
> +
>  static void iothread_instance_finalize(Object *obj)
>  {
>      IOThread *iothread = IOTHREAD(obj);
> @@ -215,6 +228,7 @@ static const TypeInfo iothread_info = {
>      .parent = TYPE_OBJECT,
>      .class_init = iothread_class_init,
>      .instance_size = sizeof(IOThread),
> +    .instance_init = iothread_instance_init,
>      .instance_finalize = iothread_instance_finalize,
>      .interfaces = (InterfaceInfo[]) {
>          {TYPE_USER_CREATABLE},
> 

ACK

Paolo