Fwd: [PATCH] spinlock spin with pause instruction

Fwd: [PATCH] spinlock spin with pause instruction

[Václav Haisman]

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Hi.

I have noticed that Cygwin's spinlock goes into heavy sleeping code
for each spin. It seems it would be a good idea to actually try to
spin a bit first. There is this 'pause' instruction which let's the
CPU make such busy loops be less busy. Here is a patch to do this.

-- 
VH

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diff --git a/winsup/cygwin/thread.cc b/winsup/cygwin/thread.cc
index 01cfd5b..56e66f1 100644
--- a/winsup/cygwin/thread.cc
+++ b/winsup/cygwin/thread.cc
@@ -1917,6 +1917,19 @@ pthread_spinlock::lock ()
 {
   pthread_t self = ::pthread_self ();
   int result = -1;
+  unsigned spins = 0;
+
+  /*
+    We want to spin using 'pause' instruction on multi-core system but we
+    want to avoid this on single-core systems.
+
+    The limit of 1000 spins is semi-arbitrary. Microsoft suggests (in their
+    InitializeCriticalSectionAndSpinCount documentation on MSDN) they are
+    using spin count limit 4000 for their heap manager critical
+    sections. Other source suggest spin count as small as 200 for fast path
+    of mutex locking.
+   */
+  unsigned const FAST_SPINS_LIMIT = wincap.cpu_count () != 1 ? 1000 : 0;
 
   do
     {
@@ -1925,8 +1938,13 @@ pthread_spinlock::lock ()
 	  set_owner (self);
 	  result = 0;
 	}
-      else if (pthread::equal (owner, self))
+      else if (unlikely(pthread::equal (owner, self)))
 	result = EDEADLK;
+      else if (spins < FAST_SPINS_LIMIT)
+        {
+          ++spins;
+          __asm__ volatile ("pause":::);
+        }
       else
 	{
 	  /* Minimal timeout to minimize CPU usage while still spinning. */

Re: [PATCH] spinlock spin with pause instruction

[Corinna Vinschen]

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On Mar 11 11:28, Václav Haisman wrote:
> Hi.
> 
> I have noticed that Cygwin's spinlock goes into heavy sleeping code
> for each spin. It seems it would be a good idea to actually try to
> spin a bit first. There is this 'pause' instruction which let's the
> CPU make such busy loops be less busy. Here is a patch to do this.

Applied.


Thanks,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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Re: Fwd: [PATCH] spinlock spin with pause instruction

[Mark Geisert]

Václav Haisman wrote:
> Hi.
>
> I have noticed that Cygwin's spinlock goes into heavy sleeping code
> for each spin. It seems it would be a good idea to actually try to
> spin a bit first. There is this 'pause' instruction which let's the
> CPU make such busy loops be less busy. Here is a patch to do this.

I wanted to try out this patch but compilation is failing on the "unlikely" 
call.  Is that a macro that needs defining or something else?
Thanks,

..mark

Re: Fwd: [PATCH] spinlock spin with pause instruction

[Václav Haisman]

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On 12.3.2016 07:25, Mark Geisert wrote:
> Václav Haisman wrote:
>> Hi.
>>
>> I have noticed that Cygwin's spinlock goes into heavy sleeping code
>> for each spin. It seems it would be a good idea to actually try to
>> spin a bit first. There is this 'pause' instruction which let's the
>> CPU make such busy loops be less busy. Here is a patch to do this.
> 
> I wanted to try out this patch but compilation is failing on the
> "unlikely" call.  Is that a macro that needs defining or something else?
> Thanks,
> 
> ..mark
> 

`unlikely()` is a macro defined in `miscfuncs.h`, right below header
guard:
<https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/miscfuncs.h;h=3a9f0258c6cbb62b8d51e96c0c5542b70659e17b;hb=HEAD>

-- 
VH


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