fuse-loop/fuse_do_work: Avoid lots of thread creations/destructions

On benchmarking metadata operations with a single threaded bonnie++
and "max_idle_threads" limited to 1, 'top' was showing suspicious
160% cpu usage.
Profiling the system with flame graphs showed that an astonishing
amount of CPU time was spent in thread creation and destruction.

After verifying the code it turned out that fuse_do_work() was
creating a new thread every time all existing idle threads
were already busy. And then just a few lines later after processing
the current request it noticed that it had created too many threads
and destructed the current thread. I.e. there was a thread
creation/destruction ping-pong.

Code is changed to only create new threads if the max number of
threads is not reached.

Furthermore, thread destruction is disabled, as creation/destruction
is expensive in general.

With this change cpu usage of passthrough_hp went from ~160% to
~80% (with different values of max_idle_threads). And bonnie
values got approximately faster by 90%. This is a with single
threaded bonnie++
bonnie++ -x 4 -q -s0  -d <path> -n 30:1:1:10 -r 0

Without this patch, using the default max_idle_threads=10 and just
a single bonnie++ the thread creation/destruction code path is not
triggered.  Just one libfuse and one application thread is just
a corner case - the requirement for the issue was just
n-application-threads >= max_idle_threads.


Signed-off-by: Bernd Schubert <bschubert@ddn.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/include/fuse_common.h b/include/fuse_common.h
index b40814f..e9d8745 100644
--- a/include/fuse_common.h
+++ b/include/fuse_common.h
@@ -857,13 +857,19 @@ struct fuse_loop_config *fuse_loop_cfg_create(void);
 void fuse_loop_cfg_destroy(struct fuse_loop_config *config);
 
 /**
- * fuse_loop_config2 setter to set the number of max idle threads.
+ * fuse_loop_config setter to set the number of max idle threads.
  */
 void fuse_loop_cfg_set_idle_threads(struct fuse_loop_config *config,
 				    unsigned int value);
 
 /**
- * fuse_loop_config2 setter to enable the clone_fd feature
+ * fuse_loop_config setter to set the number of max threads.
+ */
+void fuse_loop_cfg_set_max_threads(struct fuse_loop_config *config,
+				   unsigned int value);
+
+/**
+ * fuse_loop_config setter to enable the clone_fd feature
  */
 void fuse_loop_cfg_set_clone_fd(struct fuse_loop_config *config,
 				unsigned int value);