On Fri, Oct 29, 2010 at 06:31:21PM +0400, Alexander Zagrebin wrote: > > > I've tried the nginx with > > > disabled sendfile (the nginx.conf contains "sendfile off;"): > > > > > > $ dd if=/dev/random of=test bs=1m count=100 > > > 100+0 records in > > > 100+0 records out > > > 104857600 bytes transferred in 5.892504 secs (17795083 bytes/sec) > > > $ fetch -o /dev/null http://localhost/test > > > /dev/null 100% of 100 > > MB 41 MBps > > > $ fetch -o /dev/null http://localhost/test > > > /dev/null 100% of 100 > > MB 44 MBps > > > $ fetch -o /dev/null http://localhost/test > > > /dev/null 100% of 100 > > MB 44 MBps > > > > > > > I am really surprised with such a bad performance of sendfile. > > Will you be able to profile the issue further? > > Yes. > > > I will also try to think of some measurements. > > A transfer rate is too low for the _first_ attempt only. > Further attempts demonstrates a reasonable transfer rate. > For example, nginx with "sendfile on;": > > $ dd if=/dev/random of=test bs=1m count=100 > 100+0 records in > 100+0 records out > 104857600 bytes transferred in 5.855305 secs (17908136 bytes/sec) > $ fetch -o /dev/null http://localhost/test > /dev/null 3% of 100 MB 118 kBps > 13m50s^C > fetch: transfer interrupted > $ fetch -o /dev/null http://localhost/test > /dev/null 100% of 100 MB 39 MBps > > If there was no access to the file during some time, then everything > repeats: > The first attempt - transfer rate is too low > A further attempts - no problems > > Can you reproduce the problem on your system?
Could it be the priming of the vm object pages content ? Due to double-buffering, and (possibly false) optimization to only perform double-buffering when vm object already has some data cached, reads can prime vm object page list before file is mmapped or sendfile-ed.
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