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

From: Jesper Dangaard Brouer
Date: Wed Feb 11 2015 - 16:43:33 EST

On Wed, 11 Feb 2015 13:07:24 -0600 (CST)
Christoph Lameter <cl@xxxxxxxxx> wrote:

> 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.

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

> > Yet another lock cost.
> 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.

> > > + 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?

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

I plan to use this in softirq, where I know interrupts are on, but I
can use the less-expensive "non-save" variant local_irq_{disable,enable}.

Measurements show (x86_64 E5-2695):
* 2.860 ns cost for local_irq_{disable,enable}
* 14.840 ns cost for local_irq_save()+local_irq_restore()

Best regards,
Jesper Dangaard Brouer
MSc.CS, Sr. Network Kernel Developer at Red Hat
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