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Re: [Automake-NG] [PATCH 00/11] Several changes to parallel-tests suppor

From: Stefano Lattarini
Subject: Re: [Automake-NG] [PATCH 00/11] Several changes to parallel-tests support
Date: Wed, 09 May 2012 19:40:51 +0200

On 05/09/2012 06:03 PM, Akim Demaille wrote:
> Le 9 mai 2012 à 13:11, Stefano Lattarini a écrit :
>>> I have one worry though: what about runtime performances?  Sure,
>>> late binding provides more features, but it might be costly.  Do
>>> you have means to evaluate that we have no serious regressions?
>> Not really (even if I didn't notice any slowdown in during use of
>> the Automake testsuite); do you want me to run some measurements?
>> (And this makes me think: should we add a "performance" testsuite
>> in Automake, offering a quick and convenient way to measure how a
>> changeset impacts performances?  And if yes: would you have any
>> suggestion on how to set this up?)
> Yes, I think you should at least try to make some measurements.
I will before merging the series to 'ng/master'.

> Deeply nested Make macros can be extremely costly, especially
> because they are not lazy, this is really call by name: when used
> several times, they are evaluated several times.  There must be
> some good reason for this, but then result is longer compilation,
> which is _bad_.
> I can't find the references ATM, but I remember I read about
> some build system at Google that removes all this dynamic aspect
> that exist in Makefiles, which resulted in a much lighter impact
> of the tool itself on the build time.
>> But back to the point: notice that the pre-existing implementation of
>> the parallel testsuite harness already made quite an extensive use of
>> late bindings in the definition of $(TESTS) (to allow things like
>> overriding of TESTS and TEST_LOGS at make runtime), and this has never
>> had any negative impact on the performances.
> I was not referring especially to this, but for instance the fact
> that uniqueness of files to distribute is now on the responsability
> of $(sort) instead of being a static cost.
But that's another patch series (not yet merged into ng/master BTW,
since it still lacks a proper review):

> Again, that's just an
> example, and probably one where this added cost is probably negligible,
> as it should appear only for dist* targets.
> Consider this as a general concern.  Plain "make all" should be as fast
> as possible, so beware of macros that could be repeatedly used in
> a regular build.
I absolutely agree; in fact, I've stumbled on an issue of this kind in
my patch series "Minor refactoring and cleanup for the dist-related
targets" (referenced above), and it had been quite unpleasant.

> Tests and dist are definitely less critical (well, especially dist).
I'd say that "make check" is definitely as critical as "make all" in
this respect.

>>> Also, I don't know if it's part of your specs for automake-ng,
>>> but, performance-wise *and* from the point of view of semantics,
>>> *and* of syntax, I am very fond of this new feature of GNU Make 3.82:
>>> * New special target: .ONESHELL instructs make to invoke a single instance
>>>  of the shell and provide it with the entire recipe, regardless of how many
>>>  lines it contains.
>> Ugh, no please.  Some times it is very useful to split a recipe in distinct
>> shell invocations, and the '.ONESHELL' directive would prevent us from
>> doing so (as, AFAIK, it acts on a global level, not on a target-specific
>> level).
>> More importantly: I want Automake-NG to work with at least GNU make 3.81 as
>> well (and also GNU make 3.80, it that's not too difficult), so using a
>> feature introduced in 3.82 is a big no-no.
> While I completely agree with, and expected, your second argument,
> I confess I do not understand the first one.
Let's put it this way: if we use .ONESHELL, there is a way to easily force
the execution of a recipe to be split in different shells at user-specified
points?  If there is, I retract my objection, and start grieving with you
that we can't use such a feature.

> IMHO it should have been
> the rule from day 1 in Make that the whole recipe be evaluated by
> a single shell invocation, with set -e.  The obfuscation of the
> recipes is painful.  So I rather think that it should be the opposite:
> the much rarer cases where "it is very useful to split a recipe in distinct
> shell invocations" should be the ones that are (slightly) harder to write.
With this I can indeed agree.

> I agree that going to this model is a completely change, but hey,
> if Automake-ng does not make this move, I believe it will just
> never happen.
> I'm not trying to force anything here, I'm just grieving for something
> I really like.
OK.  And anyway, this part of the discussion is mostly moot for what
concerns Automake-NG, since we can't require 3.82 or later :-(


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