Maybe we should test our ISO if there is any performance degradation with "-Os" when we compile all the binaries (including the kernel) with this flag.
Rohit On Sun, May 22, 2011 at 11:57 AM, Rohit Joshi <[email protected]> wrote: > Hi Christophe, > > I think we had this discussion on "-Os" Vs "-O2" (size vs speed) long time > back for super-fast booting. I believe we can reach midway if Kernel > compilation is done using "-O2" and the rest is compiled with "-Os". > > From phoronix , you can see that in some cases "-Os" works better in > performance than "-O2" ( > http://global.phoronix-test-suite.com/index.php?k=profile&u=staalmannen-231-8340-1312). > The reason is that "-Os" produces small executables and the CPU takes lesser > time to move data around cache. The new intel chips, however, now contain > pre-caching to improve performance gains for "bigger" executables produced > by "-O2". So, as Alexander points out that playing with cache size > parameters may be really helpful too. > It is also possible that some binaries may not compile with "-Os". > > My take is to use a midway: compile kernel with "-O2"; compile rest (as > long as the binary compiles) with "-Os" > > Useful links (from google) for you to decide: > * http://gcc.gnu.org/onlinedocs/gcc-3.3.1/gcc/Optimize-Options.html > * > http://phoronix.com/forums/showthread.php?29090-CompilerDeathMatch-surprising-results/page2 > > Thanks > > Rohit > > On Sun, May 22, 2011 at 3:50 AM, Alexander Medvedev <[email protected]>wrote: > >> >Less perf on what ? >> >> Runtime performance. -Os disables code and data alignment (-falign-*), >> that saves space, but decreases speed on CPUs without efficient unaligned >> access. >> According to google and some testing not long ago on Phoronix and >> linux.org.ru, it is a compromise between ~10% saved space or 10% of >> performance. >> >> A small suggestion - try playing with LTO and l1/l2 cache size >> parameteres. >> > >
