On 5/27/06, Martijn van Oosterhout <kleptog@svana.org> wrote:
Given there is no way to know if you're running single threaded or not, I don't think glibc can take chances like that.
There's CPP symbol _REENTRANT for that and in run time, libc can detect call to pthread_create [1].
In any case, this isn't the issue here. Glibc doesn't do any locking unless pthread is linked in. Ofcourse, it takes a few cycles to determine that, but I don't think that'd cause a major slowdown.
You are conflicting with your previous paragraph :) Otherwise you are right - that how a libc obviously should work, right? http://marc.theaimsgroup.com/?l=glibc-alpha&m=100775741325472&w=2 http://marc.theaimsgroup.com/?l=glibc-alpha&m=112110641923178&w=2 I did a small test that does several fputc calls to /dev/null, with various workarounds: * lock.enabled is standard app. * lock.disabled calls __fsetlocking(FSETLOCKING_BYCALLER), as suggested by Ulrich Drepper. * lock.unlocked calls fputc_unlocked lock.enabled 48s lock.disabled 28s lock.unlocked 25s I attached the test, you can measure yourself. So I prepared a patch that calls __fsetlocking() in AllocateFile. Andreas, Tom could you measure if it makes any difference? -- marko [1] In the first thread I linked, there was very clever optimisation proposed using this function, that would quarantee thread-safety even without _REENTRANT. Unfortunately, event if U. Drepper changes his mind someday and fixes the locking for singe-threaded apps, it would very likely break binary compatibility with old apps, so it wont happen in the near future.
test-locking.tgz
Description: GNU Zip compressed data
Index: src/backend/storage/file/fd.c
===================================================================
RCS file: /projects/cvsroot/pgsql/src/backend/storage/file/fd.c,v
retrieving revision 1.127
diff -u -r1.127 fd.c
--- src/backend/storage/file/fd.c 5 Mar 2006 15:58:37 -0000 1.127
+++ src/backend/storage/file/fd.c 27 May 2006 16:54:36 -0000
@@ -46,6 +46,10 @@
#include <unistd.h>
#include <fcntl.h>
+#ifdef __GLIBC__
+#include <stdio_ext.h>
+#endif
+
#include "miscadmin.h"
#include "access/xact.h"
#include "storage/fd.h"
@@ -1258,6 +1262,11 @@
{
AllocateDesc *desc = &allocatedDescs[numAllocatedDescs];
+#ifdef __GLIBC__
+ /* disable glibc braindamaged locking */
+ __fsetlocking(file, FSETLOCKING_BYCALLER);
+#endif
+
desc->kind = AllocateDescFile;
desc->desc.file = file;
desc->create_subid = GetCurrentSubTransactionId();
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