Re: --enable-depend by default (was Re: [HACKERS] Patch: fix lock contention for HASHHDR.mutex)

[Andres Freund]

On 2015-12-30 10:49:27 -0500, Tom Lane wrote:
> > On Wed, Dec 30, 2015 at 5:44 PM, Andres Freund  wrote:
> >> I still maintain that --enable-depend should be on by default. We're
> >> absurdly optimizing towards saving a handful of cycles in scenarios
> >> which are usually bottlenecked by other things (build boxes spend more
> >> times on tests and such)
>
> Nonsense.  The buildfarm will simply get slower, and at least for my
> animals it's too damn slow already.

I think you're overstating the performance impact of
--enable-depend. It's just an additional output file for the compiler,
given the way we're doing it (-MMD -MP -MF $outfile). Let's make a quick test.

I've built both trees once before, to prime ccache. The test is:
for i in $(seq 1 5); do git clean -dfxq && ./configure --(dis|en)able-depend 
--quiet && time make -j4 -s;done

(coffee break)

no ccache, --disable-depend:0m55.810s, 0m58.361s, 0m58.517s, 0m58.674s, 
0m58.466s
ccache, --disable-depend:   0m5.248s,  0m5.279s,  0m5.244s,  0m5.771s,  
0m5.296s
no ccache, --enable-depend: 0m56.443s, 0m58.507s, 0m58.587s, 0m58.866s, 
0m58.429s
ccache, --enable-depend:0m5.538s,  0m5.518s,  0m5.572s,  0m5.555s,  
0m5.528s


Yes, this is on a much faster machine (my laptop) than what some
buildfarm animal are running. But given it's really just some
additional, *small*, files that are written (compiler) and read (make),
I don't see why the impact would be significantly different on slower
machines.


> We could fix that by having the
> buildfarm script automatically add --disable-depend; but if we're going
> to change this, we should roll out that script update BEFORE we change
> it, not after.

Sounds reasonable, despite the above. This isn't an urgent change, just
something to make newer developers waste less time.


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--enable-depend by default (was Re: [HACKERS] Patch: fix lock contention for HASHHDR.mutex)

[Tom Lane]

[ A change as significant as this should not be debated in a thread with
a title that suggests it's of interest to only one or two people ]

> On Wed, Dec 30, 2015 at 5:44 PM, Andres Freund  wrote:
>> I still maintain that --enable-depend should be on by default. We're
>> absurdly optimizing towards saving a handful of cycles in scenarios
>> which are usually bottlenecked by other things (build boxes spend more
>> times on tests and such)

Nonsense.  The buildfarm will simply get slower, and at least for my
animals it's too damn slow already.  We could fix that by having the
buildfarm script automatically add --disable-depend; but if we're going
to change this, we should roll out that script update BEFORE we change
it, not after.

regards, tom lane


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