linux-kernel - Re: ext4 performance falloff

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Message-ID: <87bnwdjdoj.fsf@tassilo.jf.intel.com>
Date:	Mon, 07 Apr 2014 09:40:28 -0700
From:	Andi Kleen <andi@...stfloor.org>
To:	Jan Kara <jack@...e.cz>
Cc:	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

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)

The MM uses similar techniques.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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