Re: [gentoo-user] Howto speed up compilations

[Richard Fish <bigfish@asmallpond.org>]

Zac Medico wrote:

> John J. Foster wrote:
>
>>
>> OK, this happens all the time. I search, can't find what I want, post a
>> question, search again, and there it is. This is not the "thread" I was
>> talking about, but it was right in front of me on the Gentoo Wiki.
>> http://gentoo-wiki.com/TIP_Speeding_up_portage_with_tmpfs
>>
>> Does anyone have any "tips" on these "tips"?
>>
>
> I sincerely doubt that this technique is worth the trouble because the 
> Linux virtual memory system will automatically cache frequently 
> accessed files in ram.  The tmpfs will eliminate filesystem overhead 
> and will certainly result in a reduction of total build time.  
> However, I'd go with Bruno's recommendation and just put -pipe in the 
> cflags.  For builds that work well in parallel, which most large 
> builds do, distcc is a great way to speed things up.
>
> Zac


I'm a bit dubious about this well...unless you have a huge amount of 
memory (as in, more than 2G).  But even then, if you have sufficient 
RAM, most of the source files should still be in memory as a result of 
the extraction of the source archive before compilation.  And if buffers 
get flushed/recycled as a result of the compilation, that generally 
means that the compiler needed a large amount of memory, and giving the 
compiler your RAM is a much better choice, as otherwise swap has to be 
used, and that will *kill* your compilation time!

That isn't to say that I don't think some smart and simple things beyond 
-pipe can speed up builds.  I believe /var should be it's own 
filesystem, about 5G in size, and positioned just before /usr if you 
have one, or just after / if not.  The filesystem should be either XFS 
(caches files in memory very agressively, and in fact doesn't even 
update the disk for short-lived temporary files) or reiserfs v3 (creates 
and deletes files very very quickly).

However, I can pretty much guarantee that nothing short of a 
16-processor, 16GB system (or a distcc farm, as Colin suggested) will 
allow you to "emerge -Dv --emptytree kde-meta" in 15 minutes!!!  There 
is simply no getting around the CPU and memory bandwidth required for 
compiling!

-Richard

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