linux-kernel - Re: Ctrl+C doesn't interrupt process waiting for I/O

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Message-ID: <48673544.5020408@gmail.com>
Date:	Sun, 29 Jun 2008 10:09:56 +0300
From:	Török Edwin <edwintorok@...il.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: Ctrl+C doesn't interrupt process waiting for I/O

Jeremy Fitzhardinge wrote:
> Török Edwin wrote:
>> Hi,
>>
>> I have encountered the following situation several times, but I've been
>> unable to come up with a way to reproduce this until now:
>>  - some process is keeping the disk busy (some cron job for example:
>> updatedb, chkrootkit, ...)
>>  - other processes that want to do I/O have to wait (this is normal)
>>  - I have a (I/O bound) process running in my terminal, and I want to
>> interrupt it with Ctrl+C
>>  - I type Ctrl+C several times, and the process is not interrupted for
>> several seconds (10-30 secs)
>>  - if I type Ctrl+Z, and use kill %1 the process dies faster than
>> waiting for it to react to Ctrl+C
>>
>> This issue occurs both on my x86-64 machine that uses reiserfs, and on
>> my x86 machine that uses XFS, so it doesn't seem related to the
>> underlying FS.
>> I use 2.6.25-2 and 2.6.26-rc8 now; I don't recall seeing this behaviour
>> with old kernels (IIRC I see this since 2.6.21 or 2.6.23).
>>
>> Is this intended behaviour, or should I report a bug?
>>   
>
> Yes, it's intended behaviour.  Filesystem IO syscalls are considered
> "fast" and are interruptible.  Usermode code can reasonably expect
> that file IO will never return EINTR.

Ok.

>
> That said, if a program is blocking for tens of seconds in block IO,
> then that could be a problem in itself.

In that case I don't think that a program doing heavy I/O (writeout of
100Mb+) should be able to block other processes waiting for I/O on the
same device for tens of seconds.
I am using CFQ as I/O scheduler now, I will try the other I/O schedulers
(especially deadline) and see if I get better behaviour.

Is there any documentation on the tunable values for CFQ? (in
Documentation/block there is only about anticipatory and deadline).

Best regards,
--Edwin

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