Messages in this thread |  | | From | Namhyung Kim <> | Date | Tue, 29 Dec 2020 14:35:07 +0900 | Subject | Re: [PATCH 2/3] perf tools: Skip MMAP record synthesis for kernel threads |
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On Mon, Dec 28, 2020 at 8:50 PM Jiri Olsa <jolsa@redhat.com> wrote: > > On Mon, Dec 21, 2020 at 04:00:28PM +0900, Namhyung Kim wrote: > > To synthesize information to resolve sample IPs, it needs to scan task > > and mmap info from the /proc filesystem. For each process, it > > opens (and reads) status and maps file respectively. But as kernel > > threads don't have memory maps so we can skip the maps file. > > > > To find kernel threads, check "VmPeak:" line in /proc/<PID>/status > > file. It's about the peak virtual memory usage so only user-level > > tasks have that. Also check "Threads:" line (which follows the VmPeak > > line whether or not it exists) to be sure it's read enough data - just > > in case of deeply nested pid namespaces or large number of > > supplementary groups are involved. > > how much does this save? for kernel threads the maps file is empty > and we just open the file and close > > also perhaps just stat(".....maps") and check the size would be easier?
The numbers are in the cover letter and it's around 5% on an idle machine which has mostly kernel threads. I think most of the win came from this change.
It's just to avoid those syscalls, so I wanted to use the available info in the status file.
Thanks, Namhyung
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