Re: ext4 performance falloff

[Andi Kleen <andi@firstfloor.org>]

Jan Kara <jack@suse.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@linux.intel.com -- Speaking for myself only