Messages in this thread | | | Date | Thu, 12 Feb 2015 10:43:16 +1300 | From | Jesper Dangaard Brouer <> | Subject | Re: [PATCH 2/3] slub: Support for array operations |
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On Wed, 11 Feb 2015 13:07:24 -0600 (CST) Christoph Lameter <cl@linux.com> 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 synchronized)
> > 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 start.
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 Author of http://www.iptv-analyzer.org LinkedIn: http://www.linkedin.com/in/brouer
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