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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 302239897 by mschwage on Wednesday 24th of September 2008 04:28:10 PM
Old 09-24-2008
Not sync, really- but pdflush
I should mention that I'm not really concerned so much about sync but rather on pdflush itself. ...The mechanisms they use may be different. Anyway, I notice that if I modify the pdflush behavior with these kernel parameters:
Code:
vm.dirty_writeback_centisecs=100
vm.dirty_expire_centisecs=100

...it appears to affect my dd writes negatively; that is, if I loop the dd and run it over and over again, the time for dd to complete jumps on an erratic basis.

Here's what I'm executing:
Code:
while true; do
    echo -n ">>> New File >>>"; date +%r
    /usr/bin/time -f "%E" /tmp/doit
done

and /tmp/doit contains:
dd if=/dev/zero of=/var/tmp/bigfile$i bs=1024 count=40000 oflag=nonblock 2>/dev/null

Then in another window, I am modifying sysctl parameters while the while loop is running. First, the default settings:
Code:
sysctl -w vm.dirty_writeback_centisecs=500 ; sysctl -w vm.dirty_expire_centisecs=3000
sysctl -w vm.dirty_writeback_centisecs=100 ; sysctl -w vm.dirty_expire_centisecs=100
...observe times from the while loop.  These should be more "choppy"

The fact that the dd's more frequently take longer when the centisecs parameter is shortened lead me to believe that my app is blocking when it bumps into the disk flushing. The machine is otherwise quiescent, and the size of the write (40 Meg) should not be enough to trigger pdflush. It should happen every second, based on the sysctl settings.

Note that dd is not doing direct i/o or synchronous i/o. If I explicitly set those flags, it slows the write enormously. Note too that it doesn't matter if I use the "nonblock" flag or not... i/o behavior is the same.
-Mike
P.S. We don't have a syncer daemon. Also, while running my tests above, it appears to me that the effect of sync is nearly instantaneous...
Last edited by mschwage; 09-24-2008 at 05:42 PM..
 

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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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