To: Stephen Hemminger <shemminger@vyatta.com>
Cc: Tejun Heo <tj@kernel.org>, Ingo Molnar <mingo@elte.hu>,
       Rusty Russell <rusty@rustcorp.com.au>,
       Herbert Xu <herbert@gondor.apana.org.au>, akpm@linux-foundation.org,
       hpa@zytor.com, brgerst@gmail.com, cl@linux-foundation.org,
       travis@sgi.com, linux-kernel@vger.kernel.org, steiner@sgi.com,
       hugh@veritas.com, "David S. Miller" <davem@davemloft.net>,
       netdev@vger.kernel.org,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
References: <20090115183942.GA6325@elte.hu>
	<200901170827.33729.rusty@rustcorp.com.au>
	<20090116220832.GB20653@elte.hu>
	<200901201328.24605.rusty@rustcorp.com.au>
	<20090120104022.GB29346@elte.hu> <4976B82E.1080002@kernel.org>
	<m1fxjcucp8.fsf@fess.ebiederm.org> <20090121234502.66acbcf9@s6510>
From: ebiederm@xmission.com (Eric W. Biederman)
Date: Wed, 21 Jan 2009 06:13:41 -0800
In-Reply-To: <20090121234502.66acbcf9@s6510> (Stephen Hemminger's message of "Wed\, 21 Jan 2009 23\:45\:01 +1100")
Message-ID: <m11vuwn3vu.fsf@fess.ebiederm.org>
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Subject: Re: [PATCH] percpu: add optimized generic percpu accessors
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Stephen Hemminger <shemminger@vyatta.com> writes:

> This is crap. only a small fraction of these SNMP counters are
> close enough to the hot path to deserve per-cpu treatment.

Interesting.  I was just reading af_inet.c:ipv4_mib_init_net()
to get a feel for what a large consumer of percpu counters looks
like.

I expect the patterns I saw will hold even if the base size is much
smaller.


Your statement reinforces my point that our allocations in a per cpu
area are small and our dynamic per cpu allocator is not really
optimized for small allocations.

In most places what we want are 1-5 counters, which is max 40 bytes.
And our minimum allocation is a full cache line (64-128 bytes) per
allocation.


I'm wondering if dynamic per cpu allocations should work more like
slab.  Have a set of percpu base pointers for an area, and return an
area + offset in some convenient form.

Ideally we would just have one area (allowing us to keep the base
pointer in a register), but I'm not convinced that doesn't fall down
in the face of dynamic allocation.

Eric
 
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