Package: guix-patches;
Reported by: Sarah Morgensen <iskarian <at> mgsn.dev>
Date: Sat, 19 Jun 2021 00:53:02 UTC
Severity: normal
Tags: patch
Done: Ludovic Courtès <ludo <at> gnu.org>
Bug is archived. No further changes may be made.
Message #11 received at 49107 <at> debbugs.gnu.org (full text, mbox):
From: Sarah Morgensen <iskarian <at> mgsn.dev> To: Ludovic Courtès <ludo <at> gnu.org> Cc: 49107 <at> debbugs.gnu.org Subject: Re: bug#49107: [PATCH core-updates] gnu: fontconfig: Use (locally) deterministic caching Date: Wed, 07 Jul 2021 18:17:30 -0700
Hello, Thanks for taking a look at this. Ludovic Courtès <ludo <at> gnu.org> writes: >> Fontconfig usually uses a directory's mtime as its checksum. However, when >> fontconfig detects a "broken mtime" filesystem, it will generate a directory >> checksum from the directory listing contents. This is slightly slower, as it has >> to stat all the files in all font directories. Unconditionally enabling this >> mode should get us more regular behavior. > > Nice, sounds like an improvement! > > 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: --8<---------------cut here---------------start------------->8--- % 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 --8<---------------cut here---------------end--------------->8--- And the patched fc-list: --8<---------------cut here---------------start------------->8--- % 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 --8<---------------cut here---------------end--------------->8--- 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... I suppose I should be more careful about speaking from theory! > >> I am not confident this method is fully deterministic; particular filesystem >> capabilities may still be an implicit imput in the checksums. This should only >> matter when distributing pre-generated caches. >> >> Fontconfig does honor SOURCE_DATE_EPOCH, but without a reliable mtime, >> fontconfig has no way of knowing when to update such a cache. SOURCE_DATE_EPOCH >> is now disabled for the tests because they assume working cache invalidation. > > So tests fail is we leave SOURCE_DATE_EPOCH, right? Correct. > Thanks for addressing this longstanding issue! (And apologies for the > delay… Now’s a good time to get semi-high-level changes like this one > in ‘core-updates’.) 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. -- Sarah
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