[Fontconfig] Caching strategy improvements

Tue Feb 27 02:28:01 UTC 2018

I have rebuilt 2.12.93 tonight and it appears I was mistaken. I had
attempted to replace 2.12.6 in my build chain but that must have been
reverted as 2.12.93 indeed provides ~100x improvement and builds the
cache on my system in 600ms.

I still hold this is not a "blanket you should improve it" post.
Indeed a hashmap (or any mapping between patterns and files that
allows rapid validation of non-dirty files) on disk that reuses the
patterns for files that didnt change is indeed what I suggesting from
the start. I don't see why this needs to be lock-free as the entire
structure can be atomically updated using the same mechanisms already
in use for the cache. I defer to your experience if this cache is
contended enough to warrant such a structure.

I understand this is would be a significant project which is why i
brought it to the mailing list and now that font cache build times are
in seconds for large font libraries it is indeed harder to justify.
Thank you very much for your time and sorry this all started due to a
mistake on my own part. (I hope I have not been using terms
inappropriately. but I have been using font/file interchangeably and
from your replies it appears this may have been a mistake).

--Kurt Kartaltepe

On Mon, Feb 26, 2018 at 5:26 PM, Behdad Esfahbod <behdad at behdad.org> wrote:
> On my laptop, warm fc-cache -f of over 2000 fonts takes 3.5s. So maybe worth
> checking what's taking so much time on your setup.
>
> Believe me, if we knew how to make it faster easily, we would have done. So,
> any blanket "you should improve it" has no information content whatsoever.
>
> Caching per font is not realistic.
>
> The best I can think of, requires a complete rewrite of the caching, and
> would use a single cache file that implements a lock-free hashmap on the
> disk and reuses pattern for files that didn't change. But that's a very
> significant project to undertake.
>
> On Mon, Feb 26, 2018 at 3:38 AM, Kurt Kartaltepe <kkartaltepe at gmail.com>
> wrote:
>>
>> Sorry It appears I have not been replying to the list.
>>
>> I would like to add testing on 2.12.6 before the much improved
>> performance changes was ~40s cache build times with significant disk
>> I/O. So the newly improved scanning is much appreciated but doesn't
>> solve all the issues with cache build times.
>>
>> On Mon, Feb 26, 2018 at 5:29 AM, Kurt Kartaltepe <kkartaltepe at gmail.com>
>> wrote:
>> > 2.12.93 as released on
>> > https://www.freedesktop.org/software/fontconfig/release/
>> >
>> > On Mon, Feb 26, 2018 at 5:27 AM, Behdad Esfahbod <behdad at behdad.org>
>> > wrote:
>> >> Just to make sure we are on the same page, which fontconfig version are
>> >> you
>> >> testing with?
>> >>
>> >> On Mon, Feb 26, 2018 at 3:21 AM, Kurt Kartaltepe
>> >> <kkartaltepe at gmail.com>
>> >> wrote:
>> >>>
>> >>> For clarification, I have tested with ONLY the ttf fonts on my system.
>> >>> In this case the normal 18s cache build step takes 15s. This suggests
>> >>> to me there is no significant difference between FON and TTF, as they
>> >>> made up ~21% of my fonts and removing them resulted in a proportionate
>> >>> savings. Sorry If my OP was misleadingly suggesting that FON files
>> >>> were exceptionally slow, I only meant that they may not have received
>> >>> the same improvement as TTF files which may just be my
>> >>> misunderstanding of the changes you made and lack of testing.
>> >>>
>> >>> I am concerned with why it seems acceptable to rebuild the entire
>> >>> cache when only a tiny portion of it has actually changed. Users for
>> >>> which rebuilding the cache is a significant event are those with large
>> >>> font libraries. These users are are by their very nature more likely
>> >>> to add or remove fonts from their library. It seems that this is the
>> >>> worst possible case for the current caching strategy, and *this* seems
>> >>> like an issue worth fixing.
>> >>>
>> >>> In this case if checksuming files is slower than scanning them the
>> >>> issue still stands. Why checksum files that haven't changed? Does
>> >>> fontconfig not trust filesystem metadata? It would appear directory
>> >>> change times are used in detecting when to rescan so why can this not
>> >>> be extended to files instead of the expensive checksum?
>> >>>
>> >>> FWIW an md5sum of my entire font library takes ~1s with hot caches
>> >>> which I still find unacceptable as my library is possibly
>> >>> significantly smaller and my system significantly more powerful than a
>> >>> potential user's.
>> >>>
>> >>> --Kurt Kartaltepe
>> >>>
>> >>> On Sun, Feb 25, 2018 at 9:10 PM, Behdad Esfahbod <behdad at behdad.org>
>> >>> wrote:
>> >>> > What's with fon files being slow? Please report *that* and let's fix
>> >>> > it.
>> >>> >
>> >>> > We've made scanning, like, 100x faster already. 2007 stats are
>> >>> > irrelevant.
>> >>> > Checksuming files is slower than scanning them now.
>> >>> >
>> >>> > On Sun, Feb 25, 2018 at 8:08 AM, Kurt Kartaltepe
>> >>> > <kkartaltepe at gmail.com>
>> >>> > wrote:
>> >>> >>
>> >>> >> While trying to move a project to the pango stack I noticed the
>> >>> >> native
>> >>> >> font selection backends were bad/useless on some platforms (like
>> >>> >> windows see [1]). So I opted to try and use fontconfig on all
>> >>> >> platforms as it performs outstandingly and has wonderful defaults
>> >>> >> for
>> >>> >> all platforms.
>> >>> >>
>> >>> >> However during this transition I noticed that there are some major
>> >>> >> issues with cache build speed and during investigation I see that
>> >>> >> there has recently been effort to improve the situation[2]. From
>> >>> >> what
>> >>> >> I can tell the fontconfig team has maintained that these cache
>> >>> >> issues
>> >>> >> were irrelevent for the primary fontconfig platform (linux) [3]. On
>> >>> >> linux of course the cache is global and maintained usually by font
>> >>> >> packages ensuring its up-to-date. However it was precisely this the
>> >>> >> slow cache build times that lead to package managers being required
>> >>> >> to
>> >>> >> build in additional tooling to support not rebuilding cache for
>> >>> >> every
>> >>> >> font installed [4].
>> >>> >>
>> >>> >> Anyway I hope that is enough reason to persuade you that there are
>> >>> >> substantial improvements to make to the caching strategy and they
>> >>> >> are
>> >>> >> beneficial not only for the odd platforms (osx, windows) but also
>> >>> >> for
>> >>> >> Linux.
>> >>> >>
>> >>> >> My question is if fontconfig would be receptive to
>> >>> >> building/accepting
>> >>> >> a patch modifying the caching strategy to include checkums per file
>> >>> >> instead of/in addition to per directory. Currently any change to
>> >>> >> directory (such as adding a new font) invalidates all fonts within
>> >>> >> that directory. This means for directories like the system
>> >>> >> directory
>> >>> >> it results in re scans of hundreds or more fonts. Thankfully this
>> >>> >> is
>> >>> >> faster on platforms like linux where all fonts on freetype. However
>> >>> >> this improvement in scanning did not carry over to windows with its
>> >>> >> many FNT (150 on the average install) and even on my very robust
>> >>> >> development machine building a cache for a mere 650 files takes
>> >>> >> half a
>> >>> >> minute. This might be acceptable on install of the application
>> >>> >> where
>> >>> >> we can take our time building the cache, but what happens when a
>> >>> >> user
>> >>> >> installs 1 more font? A change to cache individual file checksums
>> >>> >> would provide fontconfig a way to only require the expensive
>> >>> >> coverage
>> >>> >> check of a single font instead of the entirety of a users. I dare
>> >>> >> say
>> >>> >> with this exact change the need to use a faster less robust
>> >>> >> coverage
>> >>> >> check that made scanning freetype fonts faster may be unneeded as
>> >>> >> the
>> >>> >> number of scans required to rebuild a cache would reduced 100x on
>> >>> >> the
>> >>> >> average system or more.
>> >>> >>
>> >>> >> I'm certain such a change would be highly appreciated by all
>> >>> >> fontconfig consumers who are hoping to use its powerful feature set
>> >>> >> in
>> >>> >> a multiplatform context.
>> >>> >>
>> >>> >> --Kurt Kartaltepe
>> >>> >>
>> >>> >> [1] https://bugzilla.gnome.org/show_bug.cgi?id=162681
>> >>> >> [2]
>> >>> >>
>> >>> >>
>> >>> >> https://lists.freedesktop.org/archives/fontconfig/2017-August/005986.html
>> >>> >> [3] https://bugs.freedesktop.org/show_bug.cgi?id=64766
>> >>> >> [4]
>> >>> >>
>> >>> >>
>> >>> >> https://lists.freedesktop.org/archives/fontconfig/2007-October/002728.html
>> >>> >> _______________________________________________
>> >>> >> Fontconfig mailing list
>> >>> >> Fontconfig at lists.freedesktop.org
>> >>> >> https://lists.freedesktop.org/mailman/listinfo/fontconfig
>> >>> >
>> >>> >
>> >>> >
>> >>> >
>> >>> > --
>> >>> > behdad
>> >>> > http://behdad.org/
>> >>
>> >>
>> >>
>> >>
>> >> --
>> >> behdad
>> >> http://behdad.org/
>
>
>
>
> --
> behdad
> http://behdad.org/