On February 23, 2016 8:09:23 PM PST, Mathieu Desnoyers <[email protected]> wrote: >----- On Feb 23, 2016, at 8:36 PM, H. Peter Anvin [email protected] wrote: > >> On 02/23/2016 03:28 PM, Mathieu Desnoyers wrote: >>> Hi, >>> >>> Here is a patchset implementing a cache for the CPU number of the >>> currently running thread in user-space. >>> >>> Benchmarks comparing this approach to a getcpu based on system call >on >>> ARM show a 44x speedup. They show a 14x speedup on x86-64 compared >to >>> executing lsl from a vDSO through glibc. >>> >>> I'm added a man page in the changelog of patch 1/3, which shows an >>> example usage of this new system call. >>> >>> This series is based on v4.5-rc5, submitted for Linux 4.6. >>> >>> Feedback is welcome, >>> >> >> What is the resulting context switch overhead? > >The getcpu_cache only adds code to the thread migration path, >and to the resume notifier. The context switch path per se is >untouched. I would therefore expect the overhead on context >switch to be within the noise, except if stuff like hackbench >would be so sensitive to the size of struct task_struct that >a single extra pointer added at the end of struct task_struct >would throw off the benchmarks. > >Is that what you are concerned about ? > >Thanks, > >Mathieu
Yes, I'd like to see numbers. It is way easy to handwave small changes away, but they add up over time. Without numbers it is a bit hard to quantify the pro vs con. -- Sent from my Android device with K-9 Mail. Please excuse brevity and formatting.
