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

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.


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