[Virtio-fs] [RFC][PATCH][v3] virtiofsd: virtiofsd: Use thread pool only after certain queue depth
Greg Kurz
groug at kaod.org
Wed May 5 07:49:19 UTC 2021
On Tue, 4 May 2021 14:07:29 -0400
Vivek Goyal <vgoyal at redhat.com> wrote:
> On Tue, May 04, 2021 at 06:34:44PM +0200, Greg Kurz wrote:
> > 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.
>
> Got it. Yes, so current behavior seems to be LIFO (no-thread pool case)
> and now with this patch it will become FIFO.
>
> That LIFO behavior was not intentional. I think I was just lazy and
> did not think through it.
>
> Though we do not guarantee any ordering in terms of request ordering,
> I guess it does not hurt to process FIFO, if possible.
>
> So if you see a problem with FIFO processing, I can change it.
>
No that's fine with me since, as you say, we don't guarantee
ordering anyway and the previous LIFO behavior wasn't intentional.
> >
> > > >
> > > > > +
> > > > > + 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.
>
> Currently we offer --thread-pool-size=X option. It means a thread pool
> is created and as of now all requests are sent to thread pool.
>
Indeed we're doing this now but this is just an implementation detail
IMHO. I don't think that pushing all or only some requests to the
thread pool is relevant. I see the ---thread-pool-size=X option more
like a way to provision the maximum number of extra threads virtiofsd
is permitted to start in order to benefit from parallelism.
> Is it ok, to change that behavior (without user opting in) where upto 3
> requests are processed inline and anything more than 3 is sent to
> thread pool.
>
I think it is ok.
> Or just leave it untouched and choose new behavior as default only
> if user did not specify --thread-pool-size.
>
Not passing --thread-pool-size currently means no thread pool, i.e.
all requests are serialized. I'd rather preserve this behavior
because it clearly indicates that the user doesn't want parallelism.
Also if we use a thread pool anyway in this case, what would be its
size ?
> Thanks
> Vivek
>
Cheers,
--
Greg
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