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Full Discussion: Does sync(2) block writes until completed?
Top Forums UNIX for Advanced & Expert Users Does sync(2) block writes until completed? Post 302239838 by mschwage on Wednesday 24th of September 2008 01:59:56 PM
Old 09-24-2008
Does sync(2) block writes until completed?
Gentle readers,
I am trying to observe system behavior on our RHEL 5.2 machines. I notice that, it appears to me, based on random iterations of
Code:
dd if=/dev/zero of=/var/tmp/bigfile bs=1024 count=20000

...that if the pdflush is flushing buffers at the time of my write, my write will take about as long as
Code:
dd if=/dev/zero of=/var/tmp/bigfile bs=1024 count=20000 ; sync

This indicates to me that pdflush blocks all write()'s until the flush to disk is complete. The question is: am I correct?

Most of the time, the first dd (without the sync) is much faster than the second. By watching /proc/meminfo, I am able to see approximately when the sync initiated by pdflush takes place. This would be every 30 seconds, as my machine is not filling the disk buffers very quickly. We're talking only a few hundred kilobytes every 30 seconds.

Thanks.
-Mike
 

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LEARN ABOUT MOJAVE

glclientwaitsync

GLCLIENTWAITSYNC(3G)						    OpenGL 3.3						      GLCLIENTWAITSYNC(3G)

NAME

       glClientWaitSync - block and wait for a sync object to become signaled

C SPECIFICATION

       GLenum glClientWaitSync(GLsync sync, GLbitfield flags, GLuint64 timeout);

PARAMETERS

       sync
	   The sync object whose status to wait on.

       flags
	   A bitfield controlling the command flushing behavior.  flags may be GL_SYNC_FLUSH_COMMANDS_BIT.

       timeout
	   The timeout, specified in nanoseconds, for which the implementation should wait for sync to become signaled.

DESCRIPTION

       glClientWaitSync causes the client to block and wait for the sync object specified by sync to become signaled. If sync is signaled when
       glClientWaitSync is called, glClientWaitSync returns immediately, otherwise it will block and wait for up to timeout nanoseconds for sync
       to become signaled.

       The return value is one of four status values:

       o    GL_ALREADY_SIGNALED indicates that sync was signaled at the time that glClientWaitSync was called.

       o    GL_TIMEOUT_EXPIRED indicates that at least timeout nanoseconds passed and sync did not become signaled.

       o    GL_CONDITION_SATISFIED indicates that sync was signaled before the timeout expired.

       o    GL_WAIT_FAILED indicates that an error occurred. Additionally, an OpenGL error will be generated.

NOTES

       glClientWaitSync is available only if the GL version is 3.2 or greater.

ERRORS

       GL_INVALID_VALUE is generated if sync is not the name of an existing sync object.

       GL_INVALID_VALUE is generated if flags contains any unsupported flag.

SEE ALSO

       glFenceSync(), glIsSync() glWaitSync()

COPYRIGHT

       Copyright (C) 2010 Khronos Group. This material may be distributed subject to the terms and conditions set forth in the Open Publication
       License, v 1.0, 8 June 1999.  http://opencontent.org/openpub/.

OpenGL 3.3							    03/08/2011						      GLCLIENTWAITSYNC(3G)
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