linux-kernel - Re: ext4 performance falloff

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Message-ID: <20140407200830.GC23670@quack.suse.cz>
Date:	Mon, 7 Apr 2014 22:08:30 +0200
From:	Jan Kara <jack@...e.cz>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Jan Kara <jack@...e.cz>, Daniel J Blueman <daniel@...ascale.com>,
	Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Steffen Persvold <sp@...ascale.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>
Subject: Re: ext4 performance falloff

On Mon 07-04-14 09:40:28, Andi Kleen wrote:
> Jan Kara <jack@...e.cz> writes:
> >
> > What we really need is a counter where we can better estimate counts
> > accumulated in the percpu part of it. As the counter approaches zero, it's
> > CPU overhead will have to become that of a single locked variable but when
> > the value of counter is relatively high, we want it to be fast as the
> > percpu one. Possibly, each CPU could "reserve" part of the value in the
> > counter (by just decrementing the total value; how large that part should
> > be really needs to depend to the total value of the counter and number of
> > CPUs - in this regard we really differ from classical percpu couters) and
> > allocate/free using that part. If CPU cannot reserve what it is asked for
> > anymore, it would go and steal from parts other CPUs have accumulated,
> > returning them to global pool until it can satisfy the allocation.
> 
> That's a percpu_counter() isn't it? (or cookie jar)
  Not quite. We could use __percpu_counter_add() to set batch size for each
operation depending on the current counter value. But still we don't want
any cpu-local count to go negative (as then we cannot rely on global
counter to give us a lower bound on number of free blocks). Also stealing
from different cpu needs to be implemented...

> The MM uses similar techniques.
  Where exactly? I'd be happy to be inspired :).

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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