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Re: buffered writeback torture program

To: Jan Kara <jack@xxxxxxx>
Subject: Re: buffered writeback torture program
From: Chris Mason <chris.mason@xxxxxxxxxx>
Date: Thu, 21 Apr 2011 12:57:17 -0400
Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>, linux-fsdevel <linux-fsdevel@xxxxxxxxxxxxxxx>, linux-ext4 <linux-ext4@xxxxxxxxxxxxxxx>, xfs <xfs@xxxxxxxxxxx>, axboe <axboe@xxxxxxxxx>
In-reply-to: <20110421165529.GE4476@xxxxxxxxxxxxx>
References: <1303322378-sup-1722@think> <20110420220626.GL29872@xxxxxxxxxx> <1303383609-sup-2330@think> <1303399343-sup-9292@think> <20110421165529.GE4476@xxxxxxxxxxxxx>
User-agent: Sup/git
Excerpts from Jan Kara's message of 2011-04-21 12:55:29 -0400:
> On Thu 21-04-11 11:25:41, Chris Mason wrote:
> > Excerpts from Chris Mason's message of 2011-04-21 07:09:11 -0400:
> > > Excerpts from Vivek Goyal's message of 2011-04-20 18:06:26 -0400:
> > > > > 
> > > > > In this case the 128s spent in write was on a single 4K overwrite on a
> > > > > 4K file.
> > > > 
> > > > Chris, You seem to be doing 1MB (32768*32) writes on fsync file instead 
> > > > of 4K.
> > > > I changed the size to 4K still not much difference though.
> > > 
> > > Whoops, I had that change made locally but didn't get it copied out.
> > > 
> > > > 
> > > > Once the program has exited because of high write time, i restarted it 
> > > > and
> > > > this time I don't see high write times.
> > > 
> > > I see this for some of my runs as well.
> > > 
> > > > 
> > > > First run
> > > > ---------
> > > > # ./a.out 
> > > > setting up random write file
> > > > done setting up random write file
> > > > starting fsync run
> > > > starting random io!
> > > > write time: 0.0006s fsync time: 0.3400s
> > > > write time: 63.3270s fsync time: 0.3760s
> > > > run done 2 fsyncs total, killing random writer
> > > > 
> > > > Second run
> > > > ----------
> > > > # ./a.out 
> > > > starting fsync run
> > > > starting random io!
> > > > write time: 0.0006s fsync time: 0.5359s
> > > > write time: 0.0007s fsync time: 0.3559s
> > > > write time: 0.0009s fsync time: 0.3113s
> > > > write time: 0.0008s fsync time: 0.4336s
> > > > write time: 0.0009s fsync time: 0.3780s
> > > > write time: 0.0008s fsync time: 0.3114s
> > > > write time: 0.0009s fsync time: 0.3225s
> > > > write time: 0.0009s fsync time: 0.3891s
> > > > write time: 0.0009s fsync time: 0.4336s
> > > > write time: 0.0009s fsync time: 0.4225s
> > > > write time: 0.0009s fsync time: 0.4114s
> > > > write time: 0.0007s fsync time: 0.4004s
> > > > 
> > > > Not sure why would that happen.
> > > > 
> > > > I am wondering why pwrite/fsync process was throttled. It did not have 
> > > > any
> > > > pages in page cache and it shouldn't have hit the task dirty limits. 
> > > > Does that
> > > > mean per task dirty limit logic does not work or I am completely missing
> > > > the root cause of the problem.
> > > 
> > > I haven't traced it to see.  This test box only has 1GB of ram, so the
> > > dirty ratios can be very tight.
> > 
> > Oh, I see now.  The test program first creates the file with a big
> > streaming write.  So the task doing the streaming writes gets nailed
> > with the per-task dirty accounting because it is making a ton of dirty
> > data.
> > 
> > Then the task forks the random writer to do all the random IO.
> > 
> > Then the original pid goes back to do the fsyncs on the new file.
> > 
> > So, in the original run, we get stuffed into balance_dirty_pages because
> > the per-task limits show we've done a lot of dirties.
> > 
> > In all later runs, the file already exists, so our fsyncing process
> > hasn't done much dirtying at all.  Looks like the VM is doing something
> > sane, we just get nailed with big random IO.
>   Ok, so there isn't a problem with fsync() as such if I understand it
> right. We just block tasks in balance_dirty_pages() for a *long* time
> because it takes long time to write out that dirty IO and we make it even
> worse by trying to writeout more on behalf of the throttled task. Am I
> right? The IO-less throttling will solve this regardless of patchset we
> choose so I wouldn't be too worried about the problem now.

You're right.  With one small exception, we probably do want to rotor
out of the random buffered writes in hopes of finding some sequential IO
even with the IO-less dirty throttling.

But I definitely do expect this to get better with any of the IOless
code.

-chris

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