Re: pdflush and high load

[Andrew Morton <akpm@osdl.org>]

Andres Salomon <dilinger@debian.org> wrote:
>
> Hi,
> 
> (2nd attempt at posting this; the first one appears to have
> disappeared?)

It came through.

> Basically, what ends up happening on my system is, each pdflush process
> is handling background_writeout(), encountering congestion, and calling
> blk_congestion_wait(WRITE, HZ/10) after every loop.

Yes, the do-we-need-another-thread algorithm is rather too naive.  I could
swear it _used_ to work OK.  Maybe something changed to cause individual
threads to block for longer than they used to.  That would be an
independent problem - one pdflush instance is supposed to be able to handle
many queues (I tested one instance on 12 disks and it worked OK.  But that
was 4-5 years ago)

> My solution to the problem is to keep the blk_congestion_wait() sleep as
> uninterruptible, but add a check to the background_writeout() loop to
> check whether the current pdflush thread is actually doing anything
> useful; if it's not, just give up.

It would be better to not start a new thread in the first place.