Re: [PATCH v1 0/7] perf symbol: Reference counting, flat array storage, and LRU shrinking

[Namhyung Kim <namhyung@kernel.org>]

Hello,

On Mon, Sep 28, 2026 at 01:55:26PM -0700, Ian Rogers wrote:
> On Mon, Sep 28, 2026 at 12:45 PM Alireza Haghdoost <haghdoost@uber.com> wrote:
> >
> > On Mon, Sep 28, 2026 at 12:52 AM Ian Rogers <irogers@google.com> wrote:
> > >    annotations. Saves 48 bytes per struct symbol (two rb_nodes) and provides
> > >    cache-friendly binary search (bsearch) and callback-based iteration.
> >
> > Hi Ian,
> >
> > Thanks for posting this and for Cc'ing me. I built it on edd8a9fe2eca0,
> > the same base as the v3 series I sent last week, and compared it with
> > v3 in eager mode (today's behavior) and with --lazy-load-symbols.
> >
> > All runs use perf script --no-inline --max-stack 127
> > -F comm,tid,time,ip,sym,dso, with one warm-up and three runs; the
> > median is shown below. Peak anon is the maximum RssAnon from
> > /proc/PID/status, sampled every 10 ms.
> >
> > 1) Production: 120 s cgroup profile of a storage service on an 80-CPU
> >    host (perf record -a -g -F 99 -G <cgroup>). 54k samples, 11 DSOs,
> >    5% of lines [unknown].
> >
> >                   time     max RSS    peak anon
> >    v3 eager      4.5 s    386 MiB     314 MiB
> >    v3 lazy       3.7 s    151 MiB      80 MiB
> >    this series  95.9 s    309 MiB     275 MiB
> >
> > 2) A sample fixture from my v3 cover letter, a worst case with 73% of
> >    lines [unknown]:
> >
> >                   time     max RSS
> >    v3 eager      3.2 s    334 MiB
> >    v3 lazy       1.9 s    106 MiB
> >    this series 205.3 s    449 MiB
> >
> > Most of the time goes to reloading. On the fixture, a profile shows it
> > under map__find_symbol() -> dso__load() -> dso__load_sym(). After each
> > shrink, the first miss in a shrunk DSO re-parses its whole symtab, and
> > addresses that resolve to [unknown] miss every time.
> >
> > The peak is still set by dso__load(), since the whole symtab is
> > materialized before a shrink can run. That is the main blocker for us
> > running perf script at scale: we can't bound how much memory it will
> > use, so we have to contain it with memory.max, and then the profiling
> > job fails with an OOM kill instead of degrading.
> 
> Hi Alireza, and thanks for the feedback! I'll see what can be done
> about dso__load in v2.

Thanks for the patches and the analysis.  

I think many of this memory overhead come from the symbol string.
Probably we don't use most of them.  Then would it be nice if we can
lazy-load the strings?  Then it'd have an union of a pointer and a file
offset for symbol strings.  The find-by-name API is used rarely for
user DSOs and the kernel symbols should be fully loaded anyway.

Without the string (and the priv part), now it has a fixed size so it
should be saved in an array directly.  Probably we can add a limit there
and print it with offset when it doesn't have the name.

I'm a bit skeptical about the refcount approach here.  It may be hard to
determine when it reclaims memory.  I guess the lazy-load strings with
an array would give similar savings like Alireza's work.

Thanks,
Namhyung