linux-kernel - Re: [performance bug] kernel building regression on 64 LCPUs machine

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Date:	Mon, 28 Mar 2011 21:48:57 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, "Alex,Shi" <alex.shi@...el.com>,
	Corrado Zoccolo <czoccolo@...il.com>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	"tytso@....edu" <tytso@....edu>,
	"jaxboe@...ionio.com" <jaxboe@...ionio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Chen, Tim C" <tim.c.chen@...el.com>
Subject: Re: [performance bug] kernel building regression on 64 LCPUs
 machine

On Tue 22-03-11 13:46:06, Jeff Moyer wrote:
> Jan Kara <jack@...e.cz> writes:
> 
> > Jeff, I suppose the log files of fs_mark processes are on the same
> > device as the test directory, aren't they - that might explain the
> > flusher thread doing IO? The patch below
> 
> No.  My test environments are sane.  The file system is on a completely
> separate disk and it is unmounted and remounted in between each run.
> That should take care of any lost flushes.
  OK, I understand but let me clear out one thing. What fs_mark seems to be
doing is:
for given number of iterations { (-L option, default 1)
  fork each thread {
    start timer
    for i = 1 .. 10000 { (could be changed by option -n)
      write file i
    }
    stop timer
    for i = 1 .. 10000 {
      unlink file i
    }
    write statistics to log file
    exit
  }
  read all log files, write combined results to another log file
}

I see from blktrace data that indeed you are running more than one
iteration of the fs_mark process so the problem I was wondering about is
whether unlinking or writing of log files cannot interefere with the IO
happening while the timer is running.

Now log files are rather small so I don't really think they cause any
problem but they could be the data flusher thread writes - they are stored
in the current directory (unless you use -l option). So I my question was
aiming at where these files are stored - do you specify -l option and if
not, what is the current directory of fs_mark process in your setup?

I think unlinks could possibly cause problems and that's why I suggested we
add sync() to the parent process before it runs the next iteration...

> Did you receive my last couple of private emails commenting on the trace
> data?
  Yes, I did get them. I'll reply to them in a while.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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