[Virtio-fs] [RFC][PATCH][v3] virtiofsd: virtiofsd: Use thread pool only after certain queue depth
Greg Kurz
groug at kaod.org
Tue May 4 16:34:44 UTC 2021
On Tue, 4 May 2021 12:09:09 -0400
Vivek Goyal <vgoyal at redhat.com> wrote:
> On Tue, May 04, 2021 at 05:54:38PM +0200, Greg Kurz wrote:
> > On Tue, 4 May 2021 11:18:41 -0400
> > Vivek Goyal <vgoyal at redhat.com> wrote:
> >
> > > 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 provides a knob --thread-pool-threshold=<nr-requests>, which
> > > uses a thread pool only if number of requests on virtqueue are more
> > > than nr-requests. IOW, upto nr-requests, requests are processed inline
> > > and anything more than nr-requests is sent for processing in thread
> > > pool.
> > >
> > > I have got good results with "--thread-pool-size=16 --thread-pool-threshold=3"
> > > on my system.
> > >
> > > Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
> > > ---
> > >
> > > Test results with this patch are available here.
> > >
> > > https://github.com/rhvgoyal/virtiofs-tests/tree/master/performance-results/thread-pool-threshold/4-may-2021
> > >
> > > Changes since v2:
> > > - Renamed knob name to "--thread-pool-threshold"
> > > - Started using GQueue instead of GList to keep a list of requests
> > > received on virtqueue (Greg)
> > > - When threshold is crossed only requests above threshold are sent to
> > > thread pool. Rest are still processed inline. (Greg)
> > > ---
> > > tools/virtiofsd/fuse_i.h | 1 +
> > > tools/virtiofsd/fuse_lowlevel.c | 8 +++++++-
> > > tools/virtiofsd/fuse_virtio.c | 27 +++++++++++++--------------
> > > 3 files changed, 21 insertions(+), 15 deletions(-)
> > >
> > > Index: rhvgoyal-qemu/tools/virtiofsd/fuse_virtio.c
> > > ===================================================================
> > > --- rhvgoyal-qemu.orig/tools/virtiofsd/fuse_virtio.c 2021-05-04 09:55:30.467514740 -0400
> > > +++ rhvgoyal-qemu/tools/virtiofsd/fuse_virtio.c 2021-05-04 10:06:32.453801299 -0400
> > > @@ -445,7 +445,6 @@ err:
> > >
> > > static __thread bool clone_fs_called;
> > >
> > > -/* Process one FVRequest in a thread pool */
> > > static void fv_queue_worker(gpointer data, gpointer user_data)
> > > {
> > > struct fv_QueueInfo *qi = user_data;
> > > @@ -604,11 +603,12 @@ static void *fv_queue_thread(void *opaqu
> > > struct VuVirtq *q = vu_get_queue(dev, qi->qidx);
> > > struct fuse_session *se = qi->virtio_dev->se;
> > > GThreadPool *pool = NULL;
> > > - GList *req_list = NULL;
> > > + GQueue req_queue = G_QUEUE_INIT;
> > >
> > > if (se->thread_pool_size) {
> > > - fuse_log(FUSE_LOG_DEBUG, "%s: Creating thread pool for Queue %d\n",
> > > - __func__, qi->qidx);
> > > + fuse_log(FUSE_LOG_DEBUG, "%s: Creating thread pool for Queue %d."
> > > + " thread_pool_threshold=%u\n", __func__, qi->qidx,
> > > + se->thread_pool_threshold);
> > > pool = g_thread_pool_new(fv_queue_worker, qi, se->thread_pool_size,
> > > FALSE, NULL);
> > > if (!pool) {
> > > @@ -686,22 +686,21 @@ 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);
> > > - }
> > > + g_queue_push_tail(&req_queue, req);
> >
> > So here you replace prepend() with push_tail() but...
> >
> > > }
> > >
> > > 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);
> > > - g_list_free(req_list);
> > > - req_list = NULL;
> > > + for (int i = g_queue_get_length(&req_queue); i > 0; i--) {
> > > + FVRequest *req = g_queue_pop_head(&req_queue);
> >
> > ... this pops from the head. Isn't this reversing the order in which
> > requests are processed ? Not very harmful I guess.
>
> IIUC, g_queue_push_tail() will add each new request to tail of the queue.
> So when I pop from head, I will first pop the element which came first.
> That will be FIFO order. Am I misunderstanding it?
>
This isn't what the current code is doing in the no-thread pool case
AFAICT. It pushes requests to the head and then does foreach(), i.e.
LIFO.
> >
> > > +
> > > + if (se->thread_pool_size && i > se->thread_pool_threshold) {
> >
> > requests are pushed to the thread pool first, good. :)
> >
> > > + g_thread_pool_push(pool, req, NULL);
> > > + } else {
> > > + fv_queue_worker(req, qi);
> > > + }
> > > }
> > > }
> > >
> > > Index: rhvgoyal-qemu/tools/virtiofsd/fuse_i.h
> > > ===================================================================
> > > --- rhvgoyal-qemu.orig/tools/virtiofsd/fuse_i.h 2021-05-04 09:55:30.467514740 -0400
> > > +++ rhvgoyal-qemu/tools/virtiofsd/fuse_i.h 2021-05-04 10:04:58.737938382 -0400
> > > @@ -73,6 +73,7 @@ struct fuse_session {
> > > int vu_socketfd;
> > > struct fv_VuDev *virtio_dev;
> > > int thread_pool_size;
> > > + unsigned thread_pool_threshold;
> > > };
> > >
> > > struct fuse_chan {
> > > Index: rhvgoyal-qemu/tools/virtiofsd/fuse_lowlevel.c
> > > ===================================================================
> > > --- rhvgoyal-qemu.orig/tools/virtiofsd/fuse_lowlevel.c 2021-05-04 09:55:30.467514740 -0400
> > > +++ rhvgoyal-qemu/tools/virtiofsd/fuse_lowlevel.c 2021-05-04 10:17:27.674809167 -0400
> > > @@ -2432,6 +2432,7 @@ static const struct fuse_opt fuse_ll_opt
> > > LL_OPTION("--socket-group=%s", vu_socket_group, 0),
> > > LL_OPTION("--fd=%d", vu_listen_fd, 0),
> > > LL_OPTION("--thread-pool-size=%d", thread_pool_size, 0),
> > > + LL_OPTION("--thread-pool-threshold=%u", thread_pool_threshold, 0),
> > > FUSE_OPT_END
> > > };
> > >
> > > @@ -2452,7 +2453,11 @@ void fuse_lowlevel_help(void)
> > > " --socket-path=PATH path for the vhost-user socket\n"
> > > " --socket-group=GRNAME name of group for the vhost-user socket\n"
> > > " --fd=FDNUM fd number of vhost-user socket\n"
> > > - " --thread-pool-size=NUM thread pool size limit (default %d)\n",
> > > + " --thread-pool-size=NUM thread pool size limit (default %d)\n"
> > > + " --thread-pool-threshold=NUM\n"
> > > + " number of request threshold to\n"
> > > + " switch between inline and thread pool\n"
> > > + " processing of request\n",
> >
> > The number of requests the queue thread pops out from the virtqueue
> > looks a bit cryptic from a user point of view IMHO. Is this really
> > something we want to expose in the API ? Shouldn't this be something
> > we want to only change internally depending one some heuristics in
> > the future ?
>
> I think there are few things to consider.
>
> - We will always have capability to modify defaults internally until
> and unless user overrides that with options. So even if user does
> not specify --thread-pool-threshold, we can always use it internally
> if we can get good results across broad range of systems.
>
> - Anohter option could be that we just give user a knob to enable
> this switching behavior but number of requests are determined
> internally. We can add that knob later as well. Right now I don't
> feel like hardcoding this value "3" internally because it will
You're right but until we come up with a heuristic, if you had
good results with "3", I guess it's ok to hardcode that for now.
> vary from systems to systems and from workload to worklaod
> probably. That's why I explicitly provided this interface
> so that users can speicify this value.
>
> Having said that, this looks ugly. How a user is supposed to know
> what value they should configure by default. That makes it more
> of an interface usable in specific scenarios and not a generic
> interface.
>
That's my point. Ugly interfaces can prove to be burden in the
long term.... At least it should have x- prefix so that people
know they should probably not rely on it too much.
> So question is how to arrive at better heuristics so that virtiofsd
> itself determines right threhosld and user should not have to specify
> the threshold. Users should probably choose between whether to
> opt-in/opt-out of that new behavior.
>
I'm not even sure the user needs to know that we internally
decide to process a few requests inline when a thread pool is
available.
> That makes me think that this is a good experimental patch to show
> that we can improve performance across spectrum if we wisely choose
> between inline process and when to take help of thread pool. But we
> need to develop a method for some sort of smart heuristic to determine
> that and not leave it to user.
>
Yeah definitely.
> Thanks
> Vivek
>
> >
> > > THREAD_POOL_SIZE);
> > > }
> > >
> > > @@ -2508,6 +2513,7 @@ struct fuse_session *fuse_session_new(st
> > > se->fd = -1;
> > > se->vu_listen_fd = -1;
> > > se->thread_pool_size = THREAD_POOL_SIZE;
> > > + se->thread_pool_threshold = 0;
> > > se->conn.max_write = UINT_MAX;
> > > se->conn.max_readahead = UINT_MAX;
> > >
> > >
> >
>
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