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Date: Wed, 14 Feb 2001 23:37:59 +0100
From: Manfred Spraul <manfred@colorfullife.com>
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To: Linus Torvalds <torvalds@transmeta.com>, linux-kernel@vger.kernel.org
Subject: Re: [beta patch] SSE copy_page() / clear_page()
In-Reply-To: <3A846C84.109F1D7D@colorfullife.com> <200102092240.OAA15902@penguin.transmeta.com>
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I have another idea for sse, and this one is far safer:

only use sse prefetch, leave the string operations for the actual copy.
The prefetch operations only prefetch, don't touch the sse registers,
thus neither any reentency nor interrupt problems.

I tried the attached hack^H^H^H^Hpatch, and read(fd, buf, 4000000) from
user space got 7% faster (from 264768842 cycles to 246303748 cycles,
single cpu, noacpi, 'linux -b', fastest time from several thousand
runs).

The reason why this works is simple:

Intel Pentium III and P 4 have hardcoded "fast stringcopy" operations
that invalidate whole cachelines during write (documented in the most
obvious place: multiprocessor management, memory ordering)

The result is a very fast write, but the read is still slow.

--
	Manfred
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--- 2.4/mm/filemap.c	Wed Feb 14 10:51:42 2001
+++ build-2.4/mm/filemap.c	Wed Feb 14 22:11:44 2001
@@ -1248,6 +1248,20 @@
 		size = count;
 
 	kaddr = kmap(page);
+	if (size > 128) {
+		int i;
+		__asm__ __volatile__(
+			"mov %1, %0\n\t"
+			: "=r" (i)
+			: "r" (kaddr+offset)); /* load tlb entry */
+		for(i=0;i<size;i+=64) {
+			__asm__ __volatile__(
+				"prefetchnta (%1, %0)\n\t"
+				"prefetchnta 32(%1, %0)\n\t"
+				: /* no output */
+				: "r" (i), "r" (kaddr+offset));
+		}
+	}
 	left = __copy_to_user(desc->buf, kaddr + offset, size);
 	kunmap(page);
 	

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