Messages in this thread | | | From | Denis Vlasenko <> | Subject | Re: [RFC] x86-64: Use SSE for copy_page and clear_page | Date | Tue, 31 May 2005 11:37:00 +0300 |
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On Monday 30 May 2005 22:32, Andi Kleen wrote: > On Mon, May 30, 2005 at 12:11:23PM -0700, dean gaudet wrote: > > On Mon, 30 May 2005, dean gaudet wrote: > > > > > On Mon, 30 May 2005, Benjamin LaHaise wrote: > > > > > > > Below is a patch that uses 128 bit SSE instructions for copy_page and > > > > clear_page. This is an improvement on P4 systems as can be seen by > > > > running the test program at http://www.kvack.org/~bcrl/xmm64.c to get > > > > results like: > > > > > > it looks like the patch uses SSE2 instructions (pxor, movdqa, movntdq)... > > > if you use xorps, movaps, movntps then it works on SSE processors as well. > > > > oh and btw... on x86-64 you might want to look at using movnti with 64-bit > > registers... the memory datapath on these processors is actually 64-bits > > wide, and the 128-bit stores are broken into two 64-bit pieces internally > > anyhow. the advantage of using movnti over movntdq/movntps is that you > > don't have to save/restore the xmm register set.
And if (more like 'when', actually) next AMD CPU will have 2x128bit bus instead of 2x64bit? Revert back to XMM? > Any use of write combining for copy_page/clear_page is a bad idea. > The problem is that write combining always forces the destination > out of cache. While it gives you better microbenchmarks your real workloads > suffer because they eat lot more additional cache misses when > accessing the fresh pages. > > Don't go down that path please.
I doubt it unless real-world data will back your claim up.
I did microbenchmarking. You said it looks good in microbench but hurts real-world.
Sometime after that I made a patch which allows for switching clear/copy routines on the fly, and played a bit with real-world tests.
See http://www.thisishull.net/showthread.php?t=36562
In short, I ran forking test programs which excercise clearing and copying routines in kernel. I wasn't able to find a usage pattern where page copying using SSE non-temporal stores is a loss. Page clear was demonstrably worse, no argument about that.
If you know such usage pattern, I'd like to test it. -- vda
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