Re: Tracking actual disk write sources instead of flush thread

[Phillip Susi <psusi@ubuntu.com>]

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On 4/16/2014 10:01 AM, Matthew Wilcox wrote:
> On Tue, Apr 15, 2014 at 10:01:27PM -0400, Phillip Susi wrote:
>> A lot of disk writes, especially when they are small individual
>> files being written by several different processes, are hidden
>> behind the flush thread.  Is there no way to properly track the
>> process actually responsible for the IO, even when it is the
>> flush thread that initiates the writeout?
> 
> Correct.

Wow.  If I understand things correctly, this also means that if
process A dirties a ton of cache pages, then process B tries to write
a relatively small amount, it can end up blocking in the synchronous
flush path, and so it will appear that process B and flush are doing
all of the writes, and not process A.

That seems like a severe defect.  How can such a defect be tolerated
in this day and age?  Why does the io accounting not track how many
pages the process dirties rather than how many it actually initiates
the writeout for?


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