Date: Thu, 2 Feb 2006 19:16:00 -0800
From: Andrew Morton <akpm@osdl.org>
To: Ravikiran G Thirumalai <kiran@scalex86.org>
Cc: dada1@cosmosbay.com, davem@davemloft.net, linux-kernel@vger.kernel.org,
       shai@scalex86.org, netdev@vger.kernel.org, pravins@calsoftinc.com,
       bcrl@kvack.org
Subject: Re: [patch 3/4] net: Percpufy frequently used variables --
 proto.sockets_allocated
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Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
>
> On Fri, Jan 27, 2006 at 03:01:06PM -0800, Andrew Morton wrote:
> > Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
> > 
> > 
> > > > 
> > > > If the benchmarks say that we need to.  If we cannot observe any problems
> > > > in testing of existing code and if we can't demonstrate any benefit from
> > > > the patched code then one option is to go off and do something else ;)
> > > 
> > > We first tried plain per-CPU counters for memory_allocated, found that reads
> > > on memory_allocated was causing cacheline transfers, and then
> > > switched over to batching.  So batching reads is useful.  To avoid
> > > inaccuracy, we can maybe change percpu_counter_init to:
> > > 
> > > void percpu_counter_init(struct percpu_counter *fbc, int maxdev)
> > > 
> > > the percpu batching limit would then be maxdev/num_possible_cpus.  One would
> > > use batching counters only when both reads and writes are frequent.  With
> > > the above scheme, we would go fetch cachelines from other cpus for read
> > > often only on large cpu counts, which is not any worse than the global
> > > counter alternative, but it would still be beneficial on smaller machines,
> > > without sacrificing a pre-set deviation.  
> > > 
> > > Comments?
> > 
> > Sounds sane.
> >
> 
> Here's an implementation which delegates tuning of batching to the user.  We
> don't really need local_t at all as percpu_counter_mod is not safe against
> interrupts and softirqs  as it is.  If we have a counter which could be
> modified in process context and irq/bh context, we just have to use a
> wrapper like percpu_counter_mod_bh which will just disable and enable bottom
> halves.  Reads on the counters are safe as they are atomic_reads, and the
> cpu local variables are always accessed by that cpu only.
> 
> (PS: the maxerr for ext2/ext3 is just guesstimate)

Well that's the problem.  We need to choose production-quality values for
use in there.

> Comments?

Using num_possible_cpus() in that header file is just asking for build
errors.  Probably best to uninline the function rather than adding the
needed include of cpumask.h.

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