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* [Bug nptl/13234] New: Non-pshared condition variables are ~2-2.5x slower than pshared ones at broadcast
@ 2011-09-28 23:35 bugdal at aerifal dot cx
2012-04-29 3:02 ` [Bug nptl/13234] " bugdal at aerifal dot cx
2014-06-27 12:01 ` fweimer at redhat dot com
0 siblings, 2 replies; 3+ messages in thread
From: bugdal at aerifal dot cx @ 2011-09-28 23:35 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=13234
Bug #: 13234
Summary: Non-pshared condition variables are ~2-2.5x slower
than pshared ones at broadcast
Product: glibc
Version: unspecified
Status: NEW
Severity: normal
Priority: P2
Component: nptl
AssignedTo: drepper.fsp@gmail.com
ReportedBy: bugdal@aerifal.cx
Classification: Unclassified
Created attachment 5950
--> http://sourceware.org/bugzilla/attachment.cgi?id=5950
Test program which exhibits performance difference pshared/non-pshared.
The attached program shows NPTL's non-process-shared condition variables (which
utilize futex requeue) performing significantly worse than process-shared ones
(which simply use a broadcast futex wake). On my machine (Atom N280 dual core)
it takes ~11.7 seconds with non-pshared cond var and ~5.3 seconds with a
pshared cond var (comment/uncomment the pthread_cond_init line to change which
is used).
Of course requeue-based broadcast should scale better to huge numbers of
waiters. This test program only has 5 waiters. Still, the performance should
not be this bad. With musl libc, I get comparable performance with pshared and
non-pshared cond var (and both ways outperform NPTL, with run times around
2.5-3 seconds).
If you're unwilling to properly fix whatever's making it slow, perhaps just
using a broadcast futex wake rather than the requeue code whenever the number
of waiters is less than ~10 would be an easy "fix"...
BTW, I suspect the overly-complex sequencing code aimed at minimizing spurious
wakes, which also seems responsible for bugs 12875 and 13165, is probably part
of the problem...
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* [Bug nptl/13234] Non-pshared condition variables are ~2-2.5x slower than pshared ones at broadcast
2011-09-28 23:35 [Bug nptl/13234] New: Non-pshared condition variables are ~2-2.5x slower than pshared ones at broadcast bugdal at aerifal dot cx
@ 2012-04-29 3:02 ` bugdal at aerifal dot cx
2014-06-27 12:01 ` fweimer at redhat dot com
1 sibling, 0 replies; 3+ messages in thread
From: bugdal at aerifal dot cx @ 2012-04-29 3:02 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=13234
Rich Felker <bugdal at aerifal dot cx> changed:
What |Removed |Added
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AssignedTo|drepper.fsp at gmail dot |unassigned at sourceware
|com |dot org
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* [Bug nptl/13234] Non-pshared condition variables are ~2-2.5x slower than pshared ones at broadcast
2011-09-28 23:35 [Bug nptl/13234] New: Non-pshared condition variables are ~2-2.5x slower than pshared ones at broadcast bugdal at aerifal dot cx
2012-04-29 3:02 ` [Bug nptl/13234] " bugdal at aerifal dot cx
@ 2014-06-27 12:01 ` fweimer at redhat dot com
1 sibling, 0 replies; 3+ messages in thread
From: fweimer at redhat dot com @ 2014-06-27 12:01 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=13234
Florian Weimer <fweimer at redhat dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |fweimer at redhat dot com
Flags| |security-
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2011-09-28 23:35 [Bug nptl/13234] New: Non-pshared condition variables are ~2-2.5x slower than pshared ones at broadcast bugdal at aerifal dot cx
2012-04-29 3:02 ` [Bug nptl/13234] " bugdal at aerifal dot cx
2014-06-27 12:01 ` fweimer at redhat dot com
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