Re: [Xen-devel] [PATCH 0/6] x86: reduce paravirtualized spinlock overhead

[Juergen Gross <jgross@xxxxxxxx>]

On 04/30/2015 06:39 PM, Jeremy Fitzhardinge wrote:
> On 04/30/2015 03:53 AM, Juergen Gross wrote:
> > Paravirtualized spinlocks produce some overhead even if the kernel is
> > running on bare metal. The main reason are the more complex locking
> > and unlocking functions. Especially unlocking is no longer just one
> > instruction but so complex that it is no longer inlined.
> >
> > This patch series addresses this issue by adding two more pvops
> > functions to reduce the size of the inlined spinlock functions. When
> > running on bare metal unlocking is again basically one instruction.
> Out of curiosity, is there a measurable difference?
I did a small measurement of the pure locking functions on bare metal
without and with my patches.

spin_lock() for the first time (lock and code not in cache) dropped from
about 600 to 500 cycles.

spin_unlock() for first time dropped from 145 to 87 cycles.

spin_lock() in a loop dropped from 48 to 45 cycles.

spin_unlock() in the same loop dropped from 24 to 22 cycles.


Juergen

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