[pacman-dev] pacman cold caches performance, too much stat()ing

Nagy Gabor ngaba at bibl.u-szeged.hu
Fri Dec 11 14:00:55 EST 2009

> Hello list,
> I have been investigating the slow performance of pacman regarding
> the cold caches scenario and I'm trying to write some proof of
> concept code that improves things a lot for some cases. However I
> need your help regarding some facts I might have misunderstood, and
> any pointers to the source code you also give me would also help a
> lot. I wouldn't like to lose time changing stuff that would break
> current functionality. So here are some first questions that come to
> mind, just by using strace:
> When doing "pacman -Q blah" I can see that besides the getdents() 
> syscalls in /var/lib/pacman/local (probably caused by readdir()),
> there are also stat() and access() calls for every single
> subdirectory. Why are the last ones necessary? Isn't readdir enough?
> The same goes when doing "pacman -S blah". But in that case it
> stat()'s both 'local' and 'sync' directories, so worst case is really
> bad, it will stat() all contents of local, core, extra and
> community...
> In the case of "pacman -S" I measured that a great deal of time goes
> also to reading the "depends" files of all packages in local, please
> enlighten me what this is for. I have thought of a new way to store
> dependencies that should improve things, but I should first be sure
> it doesn't break anything and get some measurements myself.
depends files are read in order to ensure that the upgraded package
won't break any "old" dependencies.

Example: local foo requires bar=2.0 (which is installed)
Then "pacman -S bar" is not allowed (if bar in sync has different

> Thanks in advance,
> Dimitris
> P.S. Is there some option --pretend I might have missed? What I need
> is to get exactly the same actions of "pacman -S blah" or "pacman
> -Su" until the Y/N prompt, as non-root user.

-Su is even worse: We have to read all desc files in sync (to get


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