bug#24937: "deleting unused links" GC phase is too slow

[Ricardo Wurmus <rekado@elephly.net>]

Ricardo Wurmus <ricardo.wurmus@mdc-berlin.de> writes:

> Ludovic Courtès <ludo@gnu.org> writes:
>
>> Ricardo, Roel: would you be able to run that links-traversal.c from
>> <https://debbugs.gnu.org/cgi/bugreport.cgi?filename=links-traversal.c;bug=24937;msg=25;att=1>
>> on a machine with a big store, as described at
>> <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=24937#25>?
>
> I just ran this on my workstation in the office where I regularly build
> packages.  Here’s the output of “df -i /gnu”
>
>   Filesystem               Inodes   IUsed   IFree IUse% Mounted on
>   /dev/mapper/fedora-root 3301376 1098852 2202524   34% /
>
> Probably not large enough to derive conclusions about hydra’s behaviour.
>
> [I can’t run it on the shared store at the MDC because NFS performance is
> too poor.  I recently ran “guix gc --optimize” to dedupe the shared
> store (post-build deduplication is disabled since a few weeks) and it’s
> at 3,197,489 used inodes.]
>
> Here are the results of running the link-traversal code on my
> workstation:
>
> --8<---------------cut here---------------start------------->8---
> rwurmus in ~: gcc -std=gnu99 -Wall links-traversal.c  -DMODE=3
> rwurmus in ~: sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'
> rwurmus in ~: time ./a.out
> 412825 dir_entries, 107 seconds
> stat took 0 seconds
>
> real	1m47.264s
> user	0m0.214s
> sys	0m1.314s
>
> rwurmus in ~: gcc -std=gnu99 -Wall links-traversal.c  -DMODE=2
> rwurmus in ~: sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'
> rwurmus in ~: time ./a.out
> 12821 dir_entries, 107 seconds (including stat)
>
> real	1m46.475s
> user	0m0.201s
> sys	0m1.309s
> --8<---------------cut here---------------end--------------->8---

I ran this for the first time on ci.guix.gnu.org, which has a very big
store (currently at around 29TB).

df -i /gnu:

  Filesystem        Inodes     IUsed     IFree IUse% Mounted on
  /dev/sdb1      610021376 132350406 477670970   22% /gnu

I had to increase the number of MAX_ENTRIES to 135000000.

I forgot to drop caches initially.  This is the first run:

--8<---------------cut here---------------start------------->8---
root@berlin ~ [env]# gcc links-traversal.c -DMODE=3 -o links-traversal
root@berlin ~ [env]# time ./links-traversal
57079502 dir_entries, 3906 seconds
stat took 136 seconds

real    67m48.145s
user    0m59.575s
sys     2m30.065s
--8<---------------cut here---------------end--------------->8---

I aborted the run after I dropped caches after 67 minutes.

I’m going to continue testing on one of the build nodes, and I’ll try
using statx.

--
Ricardo