Re: [PATCH] syscall latency improvement #1

[Linus Torvalds]

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