RE: [PATCH] x86: Add clflush before monitor for Intel 7400 series

From: Pallipadi, Venkatesh
Date: Fri Oct 17 2008 - 16:51:54 EST

>-----Original Message-----
>From: H. Peter Anvin [mailto:hpa@xxxxxxxxx]
>Sent: Friday, October 17, 2008 1:13 PM
>To: Pallipadi, Venkatesh
>Cc: Ingo Molnar; Thomas Gleixner; linux-kernel
>Subject: Re: [PATCH] x86: Add clflush before monitor for Intel
>7400 series
>Pallipadi, Venkatesh wrote:
>> hpa,
>> Do you still have reservations about this being expensive.
>> Note that this is only done when a CPU is about to go idle and
>> The cost of clflush itself will be minimal compared to idle
>> entry + idle exit latency.
>I guess I'm a bit confused about the tradeoff of CLFLUSH versus simply
>disabling MWAIT. This is a relatively recent processor and so
>optimizing matters (if this was a P4 I would be more worried about what
>has least impact on the kernel as a whole.)

As of now this is only for a limited models of CPUs (MP CPUs)
and not a norm. It is a errata on this CPU and not
something that is going to become architectural.

>Entry and exit latency do matter (specifically, exit latency
>matters for
>longer waits and the combined entry+exit latency matters for
>short waits.)

Agreed. In this case the overhead of clflush is very low (~30 cycles)
and C1 entry+exit latency will be of the order of few hundreds of cycles for
fastest wakeup.

>On the other hand, perhaps what we need to do is to get the fix in and
>worry about performance later.
> -hpa

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