Messages in this thread | | | Date | Wed, 11 Nov 2009 15:21:26 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH RFC] [X86] performance improvement for memcpy_64.S by fast string. |
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On 11/10/2009 11:57 PM, Ma, Ling wrote: > Hi Ingo > > This program is for 64bit version, so please use 'cc -o memcpy memcpy.c -O2 -m64' >
I did some measurements with this program; I added power-of-two measurements from 1-512 bytes, plus some different alignments, and found some very interesting results:
Nehalem: memcpy_new is a win for 1024+ bytes, but *also* a win for 2-32 bytes, where the old code apparently performs appallingly bad.
memcpy_new loses in the 64-512 byte range, so the 1024 threshold is probably justified.
Core2: memcpy_new is a win for <= 512 bytes, but a lose for larger copies (possibly a win again for 16K+ copies, but those are very rare in the Linux kernel.) Surprise...
However, the difference is very small.
However, I had overlooked something much more fundamental about your patch. On Nehalem, at least *it will never get executed* (except during very early startup), because we replace the memcpy code with a jmp to memcpy_c on any CPU which has X86_FEATURE_REP_GOOD, which includes Nehalem.
So the patch is a no-op on Nehalem, and any other modern CPU.
Am I guessing that the perf numbers you posted originally were all from your user space test program?
-hpa
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