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

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Message-ID: <alpine.DEB.2.11.1502111604510.15061@gentwo.org>
Date:	Wed, 11 Feb 2015 16:06:50 -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 Thu, 12 Feb 2015, Jesper Dangaard Brouer wrote:

> > > This is quite an expensive lock with irqsave.
> >
> > Yes but we take it for all partial pages.
>
> Sure, that is good, but this might be a contention point. In a micro
> benchmark, this contention should be visible, but in real use-cases the
> given subsystem also need to spend time to use these elements before
> requesting a new batch (as long as NIC cleanup cycles don't get too
> synchronized)

Yes definitely it will be a contention point. There is no way around it.

> > Yup the page access is shared but there is one per page. Contention is
> > unlikely.
>
> Yes, contention is unlikely, but every atomic operation is expensive.
> On my system the measured cost is 8ns, and a lock/unlock does two, thus
> 16ns.  Which we then do per page freelist.

Not sure what we can do about this.

> > We can require that interrupt are off when the functions are called. Then
> > we can avoid the "save" part?
>
> Yes, we could also do so with an "_irqoff" variant of the func call,
> but given we are defining the API we can just require this from the
> start.

Allright. Lets do that then.
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