linux-kernel - Re: [patch 2/3] remove throttle_vm

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Date:	Mon, 26 Mar 2007 10:35:23 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	akpm@...ux-foundation.org
CC:	dgc@....com, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/3] remove throttle_vm_writeout()

> > 
> > Remove this function.  It's purpose was to limit the global number of
> > writeback pages from submitted by direct reclaim.  But this is equally
> > well accomplished by limited queue lengths.  When this function was
> > added, the device queues had much larger default lengths (8192
> > requests, now it's 128), causing problems.
> 
> This changelog is wrong.
> 
> Yes, the _default_ depth of CFQ was decreased.  But it is trivial for the
> user to inrcrease the queue depth again, and we'd prefer that the VM not
> shit itself in response.
> 
> Plus, more significantly, the queue is per-disk.  A system with many disks
> will easily be able to cover all memory with under-writeback pages. 
> Which is why the level of under-writeback memory is controlled at a higher
> level.
> 
> These variables are why we must not reply upon per-queue throttling in
> VFS/MM/VM.
> 
> 
> All that being said, the patch (with a new, correct changlog) is OK.  This
> is because we now control the amount of dirty memory in the machine by
> running balance_dirty_pages() at first-write-fault time.  So we can no
> longer get into the situation where all memory is dirty+writeback.

Well, sort of.  I think you are not taking SwapCache pages into
account, which just pop up out of nothing to be turned into writeback
pages.

So I think removing this function still relies on limited queues, or
some other method of limiting writeback.

Miklos
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