Bug #13941: File.exist? doesn't release gvl while waiting for (f)stat to return

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.5.0dev (2017-09-26 no_gvl_rb_stat 60025) [x86_64-linux]
• Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN

[graywolf (Gray Wolf)]

When using File.exist?, I noticed that it doesn't release gvl while waiting for system to return from (f)stat call. This causes issues on high-latency network drives, since it freezes whole mri while waiting for system to reach the other end and get (f)stat of the file in question. Attached patched resolved this issue by using rb_thread_call_without_gvl to wrap the (f)stat calls. There is performance hit, but in my opinion not significant one:

   $ ~/releaseruby_patch/bin/ruby bench.rb 
Rehearsal ------------------------------------------------
File.exist?:   0.036412   0.056616   0.093028 (  0.093075)
--------------------------------------- total: 0.093028sec

                   user     system      total        real
File.exist?:   0.042953   0.049783   0.092736 (  0.092804)

   $ ~/releaseruby_no_patch/bin/ruby bench.rb 
Rehearsal ------------------------------------------------
File.exist?:   0.056094   0.026293   0.082387 (  0.082389)
--------------------------------------- total: 0.082387sec

                   user     system      total        real
File.exist?:   0.037250   0.046702   0.083952 (  0.083956)

It's important to stress out the these numbers are over 100000 iterations. Benchmarking script is attached as well as patch to the file.c, simple blocking filesystem to test this thing and test script.

Notice that File.read is not blocking already.

[graywolf (Gray Wolf)]

• File blocking_fs.c blocking_fs.c added
• File test.rb test.rb added
• File bench.rb bench.rb added

[normalperson (Eric Wong)]

[e-mail] wrote:

https://bugs.ruby-lang.org/issues/13941

Thanks, I am OK with the minor slowdown to avoid pathological
case with slow/dead FS. I'm not sure what others think...

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed

Applied in changeset trunk|r60027.

---

Release gvl while doing (f)stat

At the moment rb_stat function is blocking. This patch changes the
behaviour to release the gvl while waiting for OS to return from
f(stat).

There is behaviour impact, but not significant (times are for 100000
iterations):

$ ~/releaseruby_patch/bin/ruby bench.rb
Rehearsal ------------------------------------------------
File.exist?: 0.036412 0.056616 0.093028 ( 0.093075)
--------------------------------------- total: 0.093028sec

               user     system      total        real

File.exist?: 0.042953 0.049783 0.092736 ( 0.092804)

$ ~/releaseruby_no_patch/bin/ruby bench.rb
Rehearsal ------------------------------------------------
File.exist?: 0.056094 0.026293 0.082387 ( 0.082389)
--------------------------------------- total: 0.082387sec

               user     system      total        real

File.exist?: 0.037250 0.046702 0.083952 ( 0.083956)

Based on the patch by Wolf [e-mail] at [ruby-core:83012],
with using rb_thread_io_blocking_region for fstat.
[Bug #13941]