linux-ext4 - Re: After unlinking a large file on ext4, the process stalls for a long time

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Message-ID: <53C7F4C2.6070804@free.fr>
Date:	Thu, 17 Jul 2014 18:07:30 +0200
From:	Mason <mpeg.blue@...e.fr>
To:	Theodore Ts'o <tytso@....edu>
CC:	Lukáš Czerner <lczerner@...hat.com>,
	Andreas Dilger <adilger@...ger.ca>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: After unlinking a large file on ext4, the process stalls for a
 long time

Theodore Ts'o wrote:

> Mason wrote:
> 
>> unlink("/mnt/hdd/xxx")                  = 0 <111.479283>
>>
>> 0.01user 111.48system 1:51.99elapsed 99%CPU (0avgtext+0avgdata 772maxresident)k
>> 0inputs+0outputs (0major+434minor)pagefaults 0swaps
> 
> ... and we're CPU bound inside the kernel.
> 
> Can you run perf so we can see exactly where we're spending the CPU?
> You're not using a journal, so I'm pretty sure what you will find is
> that we're spending all of our time in mb_free_blocks(), when it is
> updating the internal mballoc buddy bitmaps.
> 
> With a journal, this work done by mb_free_blocks() is hidden in the
> kjournal thread, and happens after the commit is completed, so it
> won't block other file system operations (other than burning some
> extra CPU on one of the multiple cores available on a typical x86
> CPU).
> 
> Also, I suspect the CPU overhead is *much* less on an x86 CPU, which
> has native bit test/set/clear instructions, whereas the MIPS
> architecture was designed by Prof. Hennessy at Stanford, who was a
> doctrinaire RISC fanatic, so there would be no bitop instructions.
> 
> Even though I'm pretty sure what we'll find, knowing exactly *where*
> in mb_free_blocks() or the function it calls would be helpful in
> knowing what we need to optimize.  So if you could try using perf
> (assuming that the perf is supported MIPS; not sure if it does) that
> would be really helpful.

Is perf "better" than oprofile? (For some metric)

I have enabled:

CONFIG_PERF_EVENTS=y
CONFIG_PROFILING=y
CONFIG_TRACEPOINTS=y
CONFIG_OPROFILE=y
CONFIG_HAVE_OPROFILE=y
CONFIG_KPROBES=y
CONFIG_KRETPROBES=y

What command-line do you suggest I run to get the output you expect?
(I'll try to get it done, but I might have to wait two weeks before
I can run these tests.)

-- 
Regards.
--
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