[Buildroot] [PATCH 1/4] pkg-download: check for already downloaded file in the download wrapper

Yann E. MORIN yann.morin.1998 at free.fr
Wed Dec 3 18:43:40 UTC 2014

Thomas, All,

On 2014-12-02 09:26 +0100, Thomas Petazzoni spake thusly:
> On Tue,  2 Dec 2014 00:24:06 +0100, Yann E. MORIN wrote:
> > Instead of repeating the same test again and again in all our download
> > rules, just delegate the check for an already downloaded file to the
> > download wrapper.
> > 
> > This clears up the path for doing the hash checks on a cached file
> > before the download.
> > 
> > Signed-off-by: "Yann E. MORIN" <yann.morin.1998 at free.fr>
> > Cc: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
> > Cc: Peter Korsgaard <jacmet at uclibc.org>
> > Cc: Gustavo Zacarias <gustavo at zacarias.com.ar>
> It has the consequence of doing another fork+exec of a shell script for
> all files, even if they have already been downloaded. I don't know if
> this is really measurable compared to the overall build time, though,
> except maybe on some cases like "make source". So maybe this is OK.

Well, if I remember correctly, make is smart enough to detect simple
commands and not spawn a shell for them, and just does the fork-exec
itself rather than calling system().

So, when counting the fork+exec you'd get:

  - simple command (program arg arg arg): make does a fork()+exec() to
    the script, the kernel sees the she-bang and the exec is a call to
    the shell to interpret the script

  - command lists (command1 || command2), make does a system(), which in
    turn does a fork()+exec() to the shell to interpret the command

So we're breaking even in that case. We even gain a call to system(), so
we should be a little tiny bit faster! ;-p

And anyway, a fork+exec is not that costly nowaday. Probably, on a
cache-cold system, the IOs will largely over-shadow the fork+exec pair I
am afraid... ;-)

Yann E. MORIN.

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