[Virtio-fs] [RFC][PATCH][v3] virtiofsd: virtiofsd: Use thread pool only after certain queue depth

[Vivek Goyal]

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);
         }
 
         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);
+
+                if (se->thread_pool_size && i > se->thread_pool_threshold) {
+                    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",
         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;