bug#49107: [PATCH core-updates] gnu: fontconfig: Use (locally) deterministic caching

["Ludovic Courtès" <ludo@gnu.org>]

Hi,

Sarah Morgensen <iskarian@mgsn.dev> skribis:

> Ludovic Courtès <ludo@gnu.org> writes:

[...]

>> Does Fontconfig stats all these files every time an application starts?
>> Did you compare ‘strace -c some app’ with and without this change, to
>> get an idea of what it costs?
>
> I *believe* it does so whenever an application calls FcFontList or such,
> which is usually on startup. I haven't done tracing with an application
> that actually uses FcFontList, but I just now ran a naive test, along
> the lines of:
>
>   $ fc-cache -rf
>   $ strace -c fc-list
>
> The old fc-list:
>
> % time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
>  28.50    0.000226           0       228       142 openat
>  22.19    0.000176           1       136        26 access
>  11.10    0.000088           0       133        70 stat
>   5.93    0.000047           0        86           close
>   5.55    0.000044           2        22           fstatfs
>   5.30    0.000042           0        54        37 readlink
>   4.54    0.000036           4         8           munmap
>   3.78    0.000030           0        70           mmap
>   2.65    0.000021           1        20           write
>   2.52    0.000020           0       104           read
>   2.52    0.000020           0        40           fstat
>   1.89    0.000015           1         8           fadvise64
>   1.77    0.000014           1        10           getpid
>   0.88    0.000007           0        10           brk
>   0.63    0.000005           0        35           getrandom
>   0.25    0.000002           2         1         1 ioctl
>   0.00    0.000000           0        15           mprotect
>   0.00    0.000000           0         2           rt_sigaction
>   0.00    0.000000           0         1           rt_sigprocmask
>   0.00    0.000000           0         1           execve
>   0.00    0.000000           0         1           arch_prctl
>   0.00    0.000000           0         1           futex
>   0.00    0.000000           0         4           getdents64
>   0.00    0.000000           0         1           set_tid_address
>   0.00    0.000000           0         1           set_robust_list
>   0.00    0.000000           0         1           prlimit64
> ------ ----------- ----------- --------- --------- ----------------
> 100.00    0.000793           0       993       276 total
>
>
> And the patched fc-list:
>
> % time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
>  17.19    0.000125           0       126        11 access
>  14.44    0.000105           0       126        52 openat
>  12.24    0.000089           1        55           mmap
>  10.32    0.000075           0        99           read
>   8.67    0.000063           4        15           mprotect
>   8.25    0.000060           0        94         6 newfstatat
>   5.50    0.000040           0        74           close
>   5.23    0.000038           0        58        40 readlink
>   4.68    0.000034          34         1           set_tid_address
>   3.30    0.000024           0        38           getrandom
>   3.16    0.000023           1        13           pread64
>   1.65    0.000012           0        26           getdents64
>   1.51    0.000011           1         9           brk
>   1.10    0.000008           4         2           rt_sigaction
>   0.83    0.000006           0         9           munmap
>   0.55    0.000004           4         1           rt_sigprocmask
>   0.55    0.000004           4         1           prlimit64
>   0.41    0.000003           3         1           arch_prctl
>   0.41    0.000003           3         1           set_robust_list
>   0.00    0.000000           0        20           write
>   0.00    0.000000           0         1         1 ioctl
>   0.00    0.000000           0         1           execve
>   0.00    0.000000           0         1           sysinfo
>   0.00    0.000000           0        11           fstatfs
>   0.00    0.000000           0         8           fadvise64
> ------ ----------- ----------- --------- --------- ----------------
> 100.00    0.000727           0       791       110 total
>
> Now that is unexpected! There are actually less stats and opens. I'm
> stumped! My profile has about 20 font packages (about 450 actual files)
> installed. For reference, for both versions, `fc-cache -rf` yields about
> 3700 stats; and `fc-cache` yeilds about 300...

Well, even better.  :-)

I went ahead and applied the patch.  ‘fontconfig-minimal’ builds fine;
‘fonconfig’ (with documentation) fails to build its PDF documentation,
but that’s not related to this change.

> I still wish we could address it "properly" by generating a cache at
> profile generation time... but it looks like fontconfig embeds the full
> paths of fonts in the cache (including username, since fonts are under
> ~/.guix-profile), so I doubt such a cache would work. I plan to try it
> eventually anyway (or perhaps someone else will), but in the meantime,
> this looks like the 80% solution.

Yeah.  It could be that each font package could contain its own cache,
and the profile hook would just assemble all these caches (provided the
file format makes it possible without too much of a headache).

Thanks!

Ludo’.