[Virtio-fs] [RFC] [PATCH][v2]: Auto switch between inline and thread pool processing

[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 is an experiment which tries to switch to between inline and
thread-pool processing.

If number of requests received on queue is less than or equal to user
specified threshold, inline processing is done. Otherwise requests are
handed over to a thread pool for processing.

A user can specify the threshold using option
"--auto-switch-threshold=<threshold>".

I have got good results with "--thread-pool-size=16 --auto-switch-threshold=3" 
on my system.
 
Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
---
Changes since v1:

Added a knob to specify auto switch threshold. This will also enable
auto switching. By default it is turned off.

---
 tools/virtiofsd/fuse_i.h        |    1 +
 tools/virtiofsd/fuse_lowlevel.c |    5 ++++-
 tools/virtiofsd/fuse_virtio.c   |   32 ++++++++++++++++++++++----------
 3 files changed, 27 insertions(+), 11 deletions(-)

Index: rhvgoyal-qemu/tools/virtiofsd/fuse_virtio.c
===================================================================
--- rhvgoyal-qemu.orig/tools/virtiofsd/fuse_virtio.c	2021-04-28 13:42:34.855145073 -0400
+++ rhvgoyal-qemu/tools/virtiofsd/fuse_virtio.c	2021-04-28 13:48:21.980538754 -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;
@@ -605,10 +614,12 @@ 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",
-                 __func__, qi->qidx);
+        fuse_log(FUSE_LOG_DEBUG, "%s: Creating thread pool for Queue %d."
+                 " auto_switch_threshold=%u\n", __func__, qi->qidx,
+                 se->auto_switch_threshold);
         pool = g_thread_pool_new(fv_queue_worker, qi, se->thread_pool_size,
                                  FALSE, NULL);
         if (!pool) {
@@ -686,22 +697,23 @@ 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 <= se->auto_switch_threshold) {
+                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;
         }
     }
 
Index: rhvgoyal-qemu/tools/virtiofsd/fuse_i.h
===================================================================
--- rhvgoyal-qemu.orig/tools/virtiofsd/fuse_i.h	2021-04-28 13:42:34.855145073 -0400
+++ rhvgoyal-qemu/tools/virtiofsd/fuse_i.h	2021-04-28 13:42:35.903188526 -0400
@@ -73,6 +73,7 @@ struct fuse_session {
     int   vu_socketfd;
     struct fv_VuDev *virtio_dev;
     int thread_pool_size;
+    unsigned auto_switch_threshold;
 };
 
 struct fuse_chan {
Index: rhvgoyal-qemu/tools/virtiofsd/fuse_lowlevel.c
===================================================================
--- rhvgoyal-qemu.orig/tools/virtiofsd/fuse_lowlevel.c	2021-04-28 13:42:34.855145073 -0400
+++ rhvgoyal-qemu/tools/virtiofsd/fuse_lowlevel.c	2021-04-28 13:42:35.904188567 -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("--auto-switch-threshold=%u", auto_switch_threshold, 0),
     FUSE_OPT_END
 };
 
@@ -2452,7 +2453,8 @@ 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"
+        "    --auto-switch-threshold=NUM     number of request threshold to switch between inline and thread pool processing of request\n",
         THREAD_POOL_SIZE);
 }
 
@@ -2508,6 +2510,7 @@ struct fuse_session *fuse_session_new(st
     se->fd = -1;
     se->vu_listen_fd = -1;
     se->thread_pool_size = THREAD_POOL_SIZE;
+    se->auto_switch_threshold = 0;
     se->conn.max_write = UINT_MAX;
     se->conn.max_readahead = UINT_MAX;