linux-kernel - Re: [PATCH 2/3] slub: Support for array operations

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Date:	Wed, 11 Feb 2015 13:07:24 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
cc:	akpm@...uxfoundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, penberg@...nel.org, iamjoonsoo@....com
Subject: Re: [PATCH 2/3] slub: Support for array operations

On Wed, 11 Feb 2015, Jesper Dangaard Brouer wrote:

> > +
> > +
> > +	spin_lock_irqsave(&n->list_lock, flags);
>
> This is quite an expensive lock with irqsave.

Yes but we take it for all partial pages.

> Yet another lock cost.

Yup the page access is shared but there is one per page. Contention is
unlikely.

> > +	spin_unlock_irqrestore(&n->list_lock, flags);
> > +	return allocated;
>
> I estimate (on my CPU) the locking cost itself is more than 32ns, plus
> the irqsave (which I've also found quite expensive, alone 14ns).  Thus,
> estimated 46ns.  Single elem slub fast path cost is 18-19ns. Thus 3-4
> elem bulking should be enough to amortized the cost, guess we are still
> good :-)

We can require that interrupt are off when the functions are called. Then
we can avoid the "save" part?

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