Messages in this thread | | | Date | Thu, 12 Nov 2009 22:04:06 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH RFC] [X86] performance improvement for memcpy_64.S by fast string. |
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On 11/12/2009 09:33 PM, Ma, Ling wrote: >> Well, so you are running cache hot and it is only a win on huge >> copies... how common are those? >> > Hi Pavel Machek > Yes, we intend to introduce movsq for huge hot size(over 1024bytes) > and avoid regression for less 1024bytes. I guess you suggest using > prefetch instruction for cold data (if I was wrong please correct me). > memcpy don't know whether data has been in cache or not, > so only when copy size is over (first level 1 cache)/2 and lower > (last level cache)/2 , prefetch will get benefit. Currently first > level cache size of most cpus is around 32KB, so it is useful for prefetch > when copy size is over 16KB, but as H. Peter Anvin mentioned in last email, > over 16KB copy in kernel is rare. >
What it sounds to me is that for Nehalem, we want to use memcpy_c for >= 1024 bytes and the old code for < 1024 bytes; for Core2 it might be the exact opposite.
Either way, whatever we do should use the appropriate static replacement mechanism.
-hpa
-- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.
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