linux-kernel - Re: Btrfs v0.16 released

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Message-Id: <20080807150809.577D82FE8BE@pmx1.sophos.com>
Date:	Thu, 7 Aug 2008 16:07:15 +0100
From:	tvrtko.ursulin@...hos.com
To:	Chris Mason <chris.mason@...cle.com>
Cc:	linux-btrfs <linux-btrfs@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Btrfs v0.16 released

Chris Mason wrote on 07/08/2008 11:34:02:

> > > * Helper threads for checksumming and other background tasks.  Most 
CPU
> > > intensive operations have been pushed off to helper threads to take
> > > advantage of SMP machines.  Streaming read and write throughput now
> > > scale to disk speed even with checksumming on.
> > 
> > Can this lead to the same Priority Inversion issues as seen with
> > kjournald?
> 
> Yes, although in general only the helper threads end up actually doing
> the IO for writes.  Unfortunately, they are almost but not quite an
> elevator.  It is tempting to try sorting the bios on the helper queues
> etc.  But I haven't done that because it gets into starvation and other
> fun.
> 
> I haven't done any real single cpu testing, it may make sense in those
> workloads to checksum and submit directly in the calling context.  But
> real single cpu boxes are harder to come by these days.

[just jumping in as a casual bystander with one remark]

For this purpose it seems booting up with limiting to one CPU should be 
sufficient.

Tvrtko


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