Date: Mon, 8 Dec 2008 14:22:26 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Theodore Tso <tytso@mit.edu>
Cc: dada1@cosmosbay.com, linux-kernel@vger.kernel.org, davem@davemloft.net,
       a.p.zijlstra@chello.nl, cmm@us.ibm.com, linux-ext4@vger.kernel.org
Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()
Message-Id: <20081208142226.c7b46f04.akpm@linux-foundation.org>
In-Reply-To: <20081208221241.GA2501@mit.edu>
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On Mon, 8 Dec 2008 17:12:41 -0500
Theodore Tso <tytso@mit.edu> wrote:

> Actually, if all popular architectures had a hardware-implemented
> atomic_t, I wonder how much ext4 really needs the percpu counter,
> especially given ext4's multiblock allocator; with ext3, given that
> each block allocation required taking a per-filesystem spin lock,
> optimizing away that spinlock was far more important for improving
> ext3's scalability.  But with the multiblock allocator, it may that
> we're going through a lot more effort than what is truly necessary.

I expect that the performance numbers for the percpu counters in the
superblock are buried away in the historical git changelogs somewhere. 
I don't recall how much difference it made.

An atomic_inc() of an fs-wide counter will have similar cost to
spin_lock() of an fs-wide lock.

If the multiblock allocator can avoid doing one atomic_inc() for each
block and can instead do atomic_add(large_value, &counter) then yes,
I'm sure that an fs-wide atomic_long_t would be OK.

Of course, similar changes should be made in trucate, etc.
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