RE: [kvm-devel] [02/17][PATCH] Implement smp_call_function_mask foria64 - V8

Zhang, Xiantao
2008-03-31T10:17:18Z

Jes Sorensen wrote:
> Zhang, Xiantao wrote:
>>> From 697d50286088e98da5ac8653c80aaa96c81abf87 Mon Sep 17 00:00:00
>>> 2001
>> From: Xiantao Zhang <xiantao.zhang@intel.com>
>> Date: Mon, 31 Mar 2008 09:50:24 +0800
>> Subject: [PATCH] KVM:IA64: Implement smp_call_function_mask for ia64
>>
>> This function provides more flexible interface for smp
>> infrastructure. Signed-off-by: Xiantao Zhang
>> <xiantao.zhang@intel.com>
>
> Hi Xiantao,
>
> I'm a little wary of the performance impact of this change. Doing a
> cpumask compare on all smp_call_function calls seems a little
> expensive. Maybe it's just noise in the big picture compared to the
> actual cost of the IPIs, but I thought I'd bring it up.
> Keep in mind that a cpumask can be fairly big these days, max NR_CPUS
> is currently 4096. For those booting a kernel with NR_CPUS at 4096 on
> a dual CPU machine, it would be a bit expensive.
>
> Why not keep smp_call_function() the way it was before, rather than
> implementing it via the call to smp_call_function_mask()?

Hi, Jes
I'm not aware of the performance impact before. If the worst case
occurs, it need 64 comparisions ? Maybe keeping old smp_call_function
is better ?
Xiantao
>
>
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