On Jan 31, 2016 11:42 PM, "Ingo Molnar" <mi...@kernel.org> wrote: > > > * r...@redhat.com <r...@redhat.com> wrote: > > > (v3: address comments raised by Frederic) > > > > Running with nohz_full introduces a fair amount of overhead. > > Specifically, various things that are usually done from the > > timer interrupt are now done at syscall, irq, and guest > > entry and exit times. > > > > However, some of the code that is called every single time > > has only ever worked at jiffy resolution. The code in > > __acct_update_integrals was also doing some unnecessary > > calculations. > > > > Getting rid of the unnecessary calculations, without > > changing any of the functionality in __acct_update_integrals > > gets us about an 11% win. > > > > Not calling the time statistics updating code more than > > once per jiffy, like is done on housekeeping CPUs and on > > all the CPUs of a non-nohz_full system, shaves off a > > further 30%. > > > > I tested this series with a microbenchmark calling > > an invalid syscall number ten million times in a row, > > on a nohz_full cpu. > > > > Run times for the microbenchmark: > > > > 4.4 3.8 seconds > > 4.5-rc1 3.7 seconds > > 4.5-rc1 + first patch 3.3 seconds > > 4.5-rc1 + first 3 patches 3.1 seconds > > 4.5-rc1 + all patches 2.3 seconds > > Another suggestion (beyond fixing the 32-bit build ;-), could you please stick > your syscall microbenchmark into 'perf bench', so that we have a standardized > way > of checking such numbers? > > In fact I'd suggest we introduce an entirely new sub-tool for system call > performance measurement - and this might be the first functionality of it. > > I've attached a quick patch that is basically a copy of 'perf bench numa' and > which measures getppid() performance (simple syscall where the result is not > cached by glibc). > > I kept the process, threading and memory allocation bits of numa.c, just in > case > we need them to measure more complex syscalls. Maybe we could keep the > threading > bits and remove the memory allocation parameters, to simplify the benchmark? > > Anyway, this could be a good base to start off on.
So much code... I'll try to take a look this week. It shouldn't be so hard to port my rdpmc-based widget over to this. --Andy