Re: [PATCH] coccicheck: process every source file at once

>
> On Tue, Oct 02, 2018 at 03:55:19PM -0400, Jeff King wrote:
> > On Tue, Oct 02, 2018 at 12:16:42PM -0700, Jacob Keller wrote:
> >
> > > make coccicheck is used in order to apply coccinelle semantic patches,
> > > and see if any of the transformations found within contrib/coccinelle/
> > > can be applied to the current code base.
> > >
> > > Pass every file to a single invocation of spatch, instead of running
> > > spatch once per source file.
> > >
> > > This reduces the time required to run make coccicheck by a significant
> > > amount of time:
> > >
> > > Prior timing of make coccicheck
> > >   real    6m14.090s
> > >   user    25m2.606s
> > >   sys     1m22.919s
> > >
> > > New timing of make coccicheck
> > >   real    1m36.580s
> > >   user    7m55.933s
> > >   sys     0m18.219s
> >
> > Yay! This is a nice result.
> >
> > It's also one of the things that Julia suggested in an earlier thread.
> > One thing I wasn't quite sure about after digging into the various
> > versions (1.0.4 on Debian stable/unstable, 1.0.6 in experimental, and
> > pre-1.0.7 at the bleeding edge) was whether the old versions would be
> > similarly helped (or work at all).
> >
> > I just replicated your results with 1.0.4.deb-3+b2 from Debian stable.
> > It's possible there are older versions floating around, but for
> > something developer-only like this, I think "in Debian stable" is a
> > reasonable enough cutoff.
>
> Linux build jobs on Travis CI run Ubuntu Trusty 14.04 LTS, and
> therefore our static analysis build job still runs 1.0.0~rc19.deb-3.
>
> This patch appears to work fine with that version, too, though note
> that I also changed the build job to don't run two parallel jobs; for
> the reason see below.
>
> > > This is nearly a 4x decrease in the time required to run make
> > > coccicheck. This is due to the overhead of restarting spatch for every
> > > file. By processing all files at once, we can amortize this startup cost
> > > across the total number of files, rather than paying it once per file.
> >
> > One minor side effect is that we lose the opportunity to parallelize
> > quite as much. However, I think the reduction in total CPU makes it well
> > worth that (and we still have 8 cocci files and growing, which should
> > keep at least 8 cores busy).
>
> One major side effect, however, is the drastically increased memory
> usage resulting from processing all files at once.  With your patch on
> top of current master:
>
>   $ for cocci in contrib/coccinelle/*.cocci ; do command time -f 'max RSS: %MkB' make ${cocci}.patch ; done
>        SPATCH contrib/coccinelle/array.cocci
>   max RSS: 1537068kB
>        SPATCH contrib/coccinelle/commit.cocci
>   max RSS: 1510920kB
>        SPATCH contrib/coccinelle/free.cocci
>   max RSS: 1393712kB
>        SPATCH contrib/coccinelle/object_id.cocci
>        SPATCH result: contrib/coccinelle/object_id.cocci.patch
>   max RSS: 1831700kB
>        SPATCH contrib/coccinelle/qsort.cocci
>   max RSS: 1384960kB
>        SPATCH contrib/coccinelle/strbuf.cocci
>   max RSS: 1395800kB
>        SPATCH contrib/coccinelle/swap.cocci
>   max RSS: 1393620kB
>        SPATCH contrib/coccinelle/xstrdup_or_null.cocci
>   max RSS: 1371716kB
>
> Without your patch the max RSS lingers around 87048kB - 101980kB,
> meaning ~15x - 18x increase
>

> This might cause quite the surprise to developers who are used to run
> 'make -jN coccicheck'; my tiny laptop came to a grinding halt with
> -j4.
>
> I think this side effect should be mentioned in the commit message.
>