Messages in this thread | | | Date | Fri, 25 Jan 2002 17:53:57 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [PATCH] syscall latency improvement #1 |
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On 26 Jan 2002, Andi Kleen wrote: > > It doesn't explain the Athlon speedups. On athlon cli is ~4 cycles.
.. and it probably serializes the instruction stream.
Look at the patch.
The _only_ thing it does for the system call path is:
- remove the "cli"
- change
cmpl $0,.. jne cmp $0,.. jne
into
movl ..,reg testl reg,reg jne
and the latter may be worth a cycle (or two, if the CPU happens to like the second form better for some other reason), but it's certainly not noticeable.
A 3.4% improvement is equivalent to something like 9 cycles, so the "cli" being faster on Athlon than on a PIII certainly explains why it's less noticeable on the Athlon, but it still makes me suspect that the _real_ cost of the cli is on the order of 8 cycles.
It should be eminently testable. Just remove the cli from the standard kernel, and do before-and-after tests.
Linus
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