Re: Surprising use of memory and time when repacking mozilla's gecko repository

[Jeff King <peff@peff.net>]

On Thu, Jul 04, 2019 at 10:13:20PM +0900, Mike Hommey wrote:

> > "public-inbox-index" (reading from git, writing to Xapian+SQLite)
> > on a dev machine got slow because core count exceeded what SATA
> > could handle and had to cap the default Xapian shard count to 3
> > by default for v2 inboxes.
>  
> AFAICT, git doesn't write from multiple threads.

Right. That's always single threaded, and the main difference there is
going to be what's in the delta base cache.

> Oh right, I forgot to mention:
> - I thought this memory usage thing was [1] but it turns out it was real
>   memory usage.
> - glibc's mallinfo stores values as int, so it's useless to know how
>   much memory was allocated when it's more than 4GB.
> - glibc's malloc_stats relies on the same int data, so while it does
>   print "in use" data, it can't print values above 4GB correctly.
> - glibc has a malloc_stats function that, according to its manual page
>   "addresses the deficiencies in malloc_stats and mallinfo", but while
>   it outputs a large XML dump, it doesn't contain anything that looks
>   remotely like the "in use" from malloc_stats.
> - So all in all, I used jemalloc to gather the "allocated" stats.

I think I explained all of the memory-usage questions in my earlier
response, but just for reference: if you have access to it, valgrind's
"massif" tool is really good for this kind of profiling. Something like:

  valgrind --tool=massif git pack-objects ...
  ms_print massif.out.*

which shows heap usage at various times, points out the snapshot with
peak usage, and shows a backtrace of the main culprits at a few
snapshots.

-Peff