netdev - Re: [PATCH] NBD: Move from a global spinlock to one lock per NBD device

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Message-ID: <CAECXXi6CLm=tOKGfq1pu5s5dSsXQc8BCpnzx-fSAaFGsUauqUg@mail.gmail.com>
Date:	Tue, 5 Feb 2013 16:31:16 -0500
From:	Paul Clements <paul.clements@...sios.com>
To:	Benjamin LaHaise <bcrl@...ck.org>
Cc:	Nicholas Thomas <nick@...emark.co.uk>, jaxboe@...ionio.com,
	netdev@...r.kernel.org, hkchu@...gle.com, davem@...emloft.net,
	eric.dumazet@...il.com
Subject: Re: [PATCH] NBD: Move from a global spinlock to one lock per NBD device

On Tue, Feb 5, 2013 at 11:22 AM, Benjamin LaHaise <bcrl@...ck.org> wrote:
> On Tue, Feb 05, 2013 at 04:14:27PM +0000, Nicholas Thomas wrote:
>> This patch is entirely based on a submission by Jerry Chu, incorporating
>> suggestions from Eric Dumazet. Modern, faster NICs make the original comment
>> on why a single lock is preferable incorrect; moving to one lock per NBD
>> device removes a performance bottleneck.
>>
>> Original patch: http://permalink.gmane.org/gmane.linux.network/207233
> ...
>> +static spinlock_t *nbd_locks __read_mostly;
> ...
>
> This is about the worst way to split up a lock possible.  Most (all?) of the
> spinlocks across nbd devices are on the same cacheline, so performance will
> be limited by the rate of cacheline bounces for the lock.  It would be far
> better to embed the spinlock in the data structures that it will be
> protecting to avoid this expensive false-sharing.

Yes, embedding this in the nbd_device would be much better, no?

--
Paul
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