[Virtio-fs] [External] [RFC] [PATCH] virtiofsd: Auto switch between inline and thread-pool processing

[Vivek Goyal]

On Sun, Apr 25, 2021 at 10:29:14AM +0800, Jiachen Zhang wrote:
> On Sat, Apr 24, 2021 at 5:11 AM Vivek Goyal <vgoyal at redhat.com> wrote:
> 
> > This is just an RFC patch for now. I am still running performance numbers
> > and see if this method of switching is good enough or not. I did one run
> > and seemed to get higher performance on deeper queue depths. There were
> > few tests where I did not match lower queue depth performance with
> > no thread pool. May be run to run variation.
> >
> >
> I think this is interesting. I guess the switch number between thread-pool
> and the inline method may change as the hardware or OS kernel changes.
> Looking forward to the results.

Hi Jiachen,

Agreed that threshold to switch between inline and thread-pool will vary
based on hardware and so many other factors. Having said that, there is
no good way to determine what's the most optimized threshold for a
particular setup. 

Maybe we will have to just go with a default which works reasonably
well and also provide a knob to user to alter that threshold. Atleast
it can be useful to experiment with different thresholds.

Vivek

> 
> Jiachen
> 
> 
> > For low queue depth workloads, inline processing works well. But for
> > high queue depth (and multiple process/thread) workloads, parallel
> > processing of thread pool can benefit.
> >
> > This patch is an experiment which tries to switch to between inline and
> > thread-pool processing. If number of requests received on queue is
> > 1, inline processing is done. Otherwise requests are handed over to
> > a thread pool for processing.
> >
> > Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
> > ---
> >  tools/virtiofsd/fuse_virtio.c |   27 +++++++++++++++++++--------
> >  1 file changed, 19 insertions(+), 8 deletions(-)
> >
> > Index: rhvgoyal-qemu/tools/virtiofsd/fuse_virtio.c
> > ===================================================================
> > --- rhvgoyal-qemu.orig/tools/virtiofsd/fuse_virtio.c    2021-04-23
> > 10:03:46.175920039 -0400
> > +++ rhvgoyal-qemu/tools/virtiofsd/fuse_virtio.c 2021-04-23
> > 10:56:37.793722634 -0400
> > @@ -446,6 +446,15 @@ err:
> >  static __thread bool clone_fs_called;
> >
> >  /* Process one FVRequest in a thread pool */
> > +static void fv_queue_push_to_pool(gpointer data, gpointer user_data)
> > +{
> > +    FVRequest *req = data;
> > +    GThreadPool *pool = user_data;
> > +
> > +    g_thread_pool_push(pool, req, NULL);
> > +}
> > +
> > +/* Process one FVRequest in a thread pool */
> >  static void fv_queue_worker(gpointer data, gpointer user_data)
> >  {
> >      struct fv_QueueInfo *qi = user_data;
> > @@ -605,6 +614,7 @@ static void *fv_queue_thread(void *opaqu
> >      struct fuse_session *se = qi->virtio_dev->se;
> >      GThreadPool *pool = NULL;
> >      GList *req_list = NULL;
> > +    int nr_req = 0;
> >
> >      if (se->thread_pool_size) {
> >          fuse_log(FUSE_LOG_DEBUG, "%s: Creating thread pool for Queue
> > %d\n",
> > @@ -686,22 +696,23 @@ static void *fv_queue_thread(void *opaqu
> >              }
> >
> >              req->reply_sent = false;
> > -
> > -            if (!se->thread_pool_size) {
> > -                req_list = g_list_prepend(req_list, req);
> > -            } else {
> > -                g_thread_pool_push(pool, req, NULL);
> > -            }
> > +            req_list = g_list_prepend(req_list, req);
> > +            nr_req++;
> >          }
> >
> >          pthread_mutex_unlock(&qi->vq_lock);
> >          vu_dispatch_unlock(qi->virtio_dev);
> >
> >          /* Process all the requests. */
> > -        if (!se->thread_pool_size && req_list != NULL) {
> > -            g_list_foreach(req_list, fv_queue_worker, qi);
> > +        if (req_list != NULL) {
> > +            if (!se->thread_pool_size || nr_req <= 2) {
> > +                g_list_foreach(req_list, fv_queue_worker, qi);
> > +            } else  {
> > +                g_list_foreach(req_list, fv_queue_push_to_pool, pool);
> > +            }
> >              g_list_free(req_list);
> >              req_list = NULL;
> > +            nr_req = 0;
> >          }
> >      }
> >
> >
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> >