Re: [PATCH v3 0/8] Add io_dir to avoid memory overhead from opendir

[Ian Rogers]

On Mon, Feb 24, 2025 at 4:30 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> On Mon, Feb 24, 2025 at 04:28:24PM -0800, Namhyung Kim wrote:
> > Hi Ian,
> >
> > On Fri, Feb 21, 2025 at 10:10:05PM -0800, Ian Rogers wrote:
> > > glibc's opendir allocates a minimum of 32kb, when called recursively
> > > for a directory tree the memory consumption can add up - nearly 300kb
> > > during perf start-up when processing modules. Add a stack allocated
> > > variant of readdir sized a little more than 1kb
> >
> > It's still small and hard to verify.  I've run the following command
> > before and after the change but didn't see a difference.
> >
> >   $ sudo time -f %Mk ./perf record -a true
> >   [ perf record: Woken up 1 times to write data ]
> >   [ perf record: Captured and wrote 1.757 MB perf.data (563 samples) ]
> >   74724k
> >
> > According to man time(1), %M is for max RSS.
>
> But anyway, it looks ok and build is fine now.

Thanks for the testing! So doing a regular build I could repeat what
you saw - basically the opendir isn't contributing to maxrss as the
BPF handling and so gets lost. Doing a minimal static build, that
loses BPF support, things were clearer but not as good as I'd
originally measured, 10880k reduced to 10696k - a 184k saving (raw
data below). Perhaps opendir got better or perhaps there are fewer
kernel modules. I tried heaptrack but unfortunately it wasn't able to
instrument the allocations in glibc's allocdir function (it reported
0). Originally heaptrack showing opendir allocations were significant
for `perf record` was what led me to this code. At the moment BPF
event synthesis and topdown event checking look particularly
expensive.

Thanks,
Ian

Before:
$ sudo /bin/time -f %Mk /tmp/perf/perf record -a true
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 2.489 MB perf.data (107 samples) ]
10880k
$ sudo /bin/time -f %Mk /tmp/perf/perf record -a true
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 2.489 MB perf.data (106 samples) ]
10880k
$ sudo /bin/time -f %Mk /tmp/perf/perf record -a true
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 2.489 MB perf.data (109 samples) ]
10880k
$ sudo /bin/time -f %Mk /tmp/perf/perf record -a true
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 2.489 MB perf.data (102 samples) ]
10880k
$ sudo /bin/time -f %Mk /tmp/perf/perf record -a true
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 2.489 MB perf.data (106 samples) ]
11008k

After:
$ sudo /bin/time -f %Mk /tmp/perf/perf record -a true
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 2.473 MB perf.data (106 samples) ]
10820k
$ sudo /bin/time -f %Mk /tmp/perf/perf record -a true
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 2.473 MB perf.data (109 samples) ]
10696k
$ sudo /bin/time -f %Mk /tmp/perf/perf record -a true
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 2.473 MB perf.data (98 samples) ]
10696k
$ sudo /bin/time -f %Mk /tmp/perf/perf record -a true
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 2.489 MB perf.data (98 samples) ]
10696k
$ sudo /bin/time -f %Mk /tmp/perf/perf record -a true
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 2.490 MB perf.data (110 samples) ]
10696k