On Mon, 17 Feb 2025 18:00:22 +0000 "Colin King (gmail)" <colin.i.king@xxxxxxxxx> wrote:
> fOn 17/02/2025 17:58, Matthew Wilcox wrote:
> > On Mon, Feb 17, 2025 at 05:09:34PM +0000, Colin Ian King wrote:
> >> Adding an unlikely() hint on the masked start comparison error
> >> return path improves run-time performance of the mincore system call.
> >>
> >> Benchmarking on an i9-12900 shows an improvement of 7ns on mincore calls
> >> on a 256KB mmap'd region where 50% of the pages we resident.
> >>
> >> Results based on running 20 tests with turbo disabled (to reduce
> >> clock freq turbo changes), with 10 second run per test and comparing
> >> the number of mincores calls per second. The % standard deviation of
> >> the 20 tests was ~0.10%, so results are reliable.
> >
> > I think you've elided _just_ enough information here that nobody can
> > judge whether your stats skills are any good ;-) You've told us 7ns
> > (per call, presumably) and you've told us 0.10% standard deviation,
> > but you haven't told us how long the syscall takes, so nobody can tell
> > whether 7ns is within 0.10% or not ;-)
>
> Ugh, my bad.
>
> Improvement was from ~970 down to 963 ns, so small ~0.7% improvement.
>
It actually doesn't change the generated code:
hp2:/usr/src/25> diff -u mm/mincore.lst.old mm/mincore.lst
--- mm/mincore.lst.old 2025-02-17 19:11:34.093727411 -0800
+++ mm/mincore.lst 2025-02-17 19:12:59.797009056 -0800
@@ -1563,7 +1563,7 @@
start = untagged_addr(start);
/* Check the start address: needs to be page-aligned.. */
- if (start & ~PAGE_MASK)
+ if (unlikely(start & ~PAGE_MASK))
b27: 31 ff xor %edi,%edi
asm (ALTERNATIVE("",
b29: 90 nop
