Messages in this thread | | | From | (Linus Torvalds) | Subject | Re: gcc -O2 vs gcc -Os performance | Date | Thu, 6 Feb 2003 22:12:51 +0000 (UTC) |
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In article <263740000.1044563891@[10.10.2.4]>, Martin J. Bligh <mbligh@aracnet.com> wrote: >>> All done with gcc-2.95.4 (Debian Woody). These machines (16x NUMA-Q) have >>> 700MHz P3 Xeons with 2Mb L2 cache ... -Os might fare better on celeron >>> with a puny cache if someone wants to try that out >> >> gcc 3.2 is a lot smarter about -Os and it makes a very big size >> difference according to the numbers the from the ACPI guys. >> >> Im not sure testing with a gcc from the last millenium is useful 8) > >Still no use. >/me throws gcc-3.2 in the trash can. > >2901299 vmlinux.O2 >2667827 vmlinux.Os
Well, Os is certainly smaller. One thing to look out for is that microbenchmarks for kernels are usually the _worst_ things to test with Os.
That's since a large part of the premise of the -Os speed advantage is that it is better for icache (usually not an issue for microbenchmarks) and that it is better for load/startup times (generally not a huge issue for kernels, since the real startup costs of kernels tend to be entirely elsewhere).
So I suspect -Os tends to be more appropriate for user-mode code, and especially code with low repeat rates. Possibly the "low repeat rate" thing ends up being true of certain kernel subsystems too.
Think of it this way: if you win 10% in size, you're likely to map and load 10% less code pages at run-time. Which is not a big issue for traditional data-centric loads, but can be a _huge_ deal for things like GUI programs etc where there is often more code than data.
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