llvm dev - Jul 2017 - CMake dependencies and building LLVM

Hi everyone,
I have felt for quite some time that during my regular LLVM workflow, CMake
recompiles a lot of files after I update to the latest ToT (which I do more
than once per week). Of course, this wasn't really based on any real data,
just felt like every time I update (even if it's a day or two later), we
end up recompiling everything. For a while I assumed that patches touch a
few headers that are used all over the place or something like that, but
this was happening with what seems like unexpected frequency.

So what I've done is do a few update/rebuild cycles for revisions that are
200 apart, then 100, then 50, then 25. And it seems that quite commonly, we
recompile more than 50% of the source with just a few landed revisions.
I've included the numbers below (and I do realize that this is a fairly
small sample).

If this seems reasonable to most of the community, so be it - I work on a
large server that chews through this stuff pretty quickly so I can
certainly live with it. On the other hand, if this seems way higher than
what you expect, it could very well be that something is wrong in my setup
and I'd appreciate some pointers about how to go about investigating this.
Finally, if others are seeing this and we as a community do feel that
something can/should be done about this (although I don't know what that
might be), perhaps we can start that discussion.

Setup: CMake with -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -G
Ninja

The numbers here are the total numbers of targets ninja reports as having
rebuilt. Ninja is invoked only `-j <N>` for parameters (i.e. not check-all
or any specific targets).

RevFrm, RevTo,  NumNinjaObjsRebuilt
300000, 300200, 2957
300200, 300400, 2692
300400, 300600, 3001
300600, 300800, 2877
300800, 301000, 3003
301000, 301200, 2750
301200, 301400, 2579
301400, 301600, 2709
301600, 301700, 2325
301700, 301800, 2357
301800, 301900, 2667
301900, 302000, 2466
302000, 302100, 2499
302100, 302200, 2063
302200, 302250, 2884
302250, 302300, 2007
302300, 302350, 2209
302350, 302400, 737
302400, 302450, 2398
302450, 302500, 2856
302500, 302550, 2090
302550, 302600, 2866
302600, 302625, 1786
302625, 302650, 1966
302650, 302675, 1583
302675, 302700, 307
302700, 302725, 2246
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170707/033d1a2a/attachment.html>

I'm /guessing/ this is probably pretty normal - but likely the best
solution would be to get down to granularity of 1 commit in each of those
ranges, to see if it's really justified that the change causes so much
recompilation (ie: do all the things that got compiled really depend on the
modified file)

Some of this might be tablegen recompiling - but I /think/ that ninja has
some support/seems to do the right thing when the resulting tablegen binary
isn't different from previously (or maybe when it doesn't generate
different output - this would be better) & doesn't cause everything to
rebuild (you'll notice Ninja sometimes starts with a very high estimate
(1000+) of targets to rebuild, but the estimate is adjusted down to only a
couple of hundred in many cases probably due to this.

On Fri, Jul 7, 2017 at 6:47 AM Nemanja Ivanovic via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi everyone,
> I have felt for quite some time that during my regular LLVM workflow,
> CMake recompiles a lot of files after I update to the latest ToT (which I
> do more than once per week). Of course, this wasn't really based on any
> real data, just felt like every time I update (even if it's a day or
two
> later), we end up recompiling everything. For a while I assumed that
> patches touch a few headers that are used all over the place or something
> like that, but this was happening with what seems like unexpected
frequency.
>
> So what I've done is do a few update/rebuild cycles for revisions that
are
> 200 apart, then 100, then 50, then 25. And it seems that quite commonly, we
> recompile more than 50% of the source with just a few landed revisions.
> I've included the numbers below (and I do realize that this is a fairly
> small sample).
>
> If this seems reasonable to most of the community, so be it - I work on a
> large server that chews through this stuff pretty quickly so I can
> certainly live with it. On the other hand, if this seems way higher than
> what you expect, it could very well be that something is wrong in my setup
> and I'd appreciate some pointers about how to go about investigating
this.
> Finally, if others are seeing this and we as a community do feel that
> something can/should be done about this (although I don't know what
that
> might be), perhaps we can start that discussion.
>
> Setup: CMake with -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON
> -G Ninja
>
> The numbers here are the total numbers of targets ninja reports as having
> rebuilt. Ninja is invoked only `-j <N>` for parameters (i.e. not
check-all
> or any specific targets).
>
> RevFrm, RevTo,  NumNinjaObjsRebuilt
> 300000, 300200, 2957
> 300200, 300400, 2692
> 300400, 300600, 3001
> 300600, 300800, 2877
> 300800, 301000, 3003
> 301000, 301200, 2750
> 301200, 301400, 2579
> 301400, 301600, 2709
> 301600, 301700, 2325
> 301700, 301800, 2357
> 301800, 301900, 2667
> 301900, 302000, 2466
> 302000, 302100, 2499
> 302100, 302200, 2063
> 302200, 302250, 2884
> 302250, 302300, 2007
> 302300, 302350, 2209
> 302350, 302400, 737
> 302400, 302450, 2398
> 302450, 302500, 2856
> 302500, 302550, 2090
> 302550, 302600, 2866
> 302600, 302625, 1786
> 302625, 302650, 1966
> 302650, 302675, 1583
> 302675, 302700, 307
> 302700, 302725, 2246
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170707/5279e86b/attachment.html>

> On 7 Jul 2017, at 17:12, David Blaikie via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> I'm /guessing/ this is probably pretty normal - but likely the best
solution would be to get down to granularity of 1 commit in each of those
ranges, to see if it's really justified that the change causes so much
recompilation (ie: do all the things that got compiled really depend on the
modified file)
> 
> Some of this might be tablegen recompiling - but I /think/ that ninja has
some support/seems to do the right thing when the resulting tablegen binary
isn't different from previously (or maybe when it doesn't generate
different output - this would be better) & doesn't cause everything to
rebuild (you'll notice Ninja sometimes starts with a very high estimate
(1000+) of targets to rebuild, but the estimate is adjusted down to only a
couple of hundred in many cases probably due to this.
I can confirm that tablegen-erated files do the right thing and that it's
based on the output of tablegen. Our cmake scripts do it by having llvm-tblgen
write to a temporary file and then use 'cmake -E copy_if_different' to
update the real one when necessary. Once ninja has run the copy_if_different
command it re-checks the dependencies and culls the ones it doesn't need.
> On Fri, Jul 7, 2017 at 6:47 AM Nemanja Ivanovic via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> Hi everyone,
> I have felt for quite some time that during my regular LLVM workflow, CMake
recompiles a lot of files after I update to the latest ToT (which I do more than
once per week). Of course, this wasn't really based on any real data, just
felt like every time I update (even if it's a day or two later), we end up
recompiling everything. For a while I assumed that patches touch a few headers
that are used all over the place or something like that, but this was happening
with what seems like unexpected frequency.
> 
> So what I've done is do a few update/rebuild cycles for revisions that
are 200 apart, then 100, then 50, then 25. And it seems that quite commonly, we
recompile more than 50% of the source with just a few landed revisions. I've
included the numbers below (and I do realize that this is a fairly small
sample).
> 
> If this seems reasonable to most of the community, so be it - I work on a
large server that chews through this stuff pretty quickly so I can certainly
live with it. On the other hand, if this seems way higher than what you expect,
it could very well be that something is wrong in my setup and I'd appreciate
some pointers about how to go about investigating this. Finally, if others are
seeing this and we as a community do feel that something can/should be done
about this (although I don't know what that might be), perhaps we can start
that discussion.
> 
> Setup: CMake with -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -G
Ninja
> 
> The numbers here are the total numbers of targets ninja reports as having
rebuilt. Ninja is invoked only `-j <N>` for parameters (i.e. not check-all
or any specific targets).
> 
> RevFrm, RevTo,  NumNinjaObjsRebuilt
> 300000, 300200, 2957
> 300200, 300400, 2692
> 300400, 300600, 3001
> 300600, 300800, 2877
> 300800, 301000, 3003
> 301000, 301200, 2750
> 301200, 301400, 2579
> 301400, 301600, 2709
> 301600, 301700, 2325
> 301700, 301800, 2357
> 301800, 301900, 2667
> 301900, 302000, 2466
> 302000, 302100, 2499
> 302100, 302200, 2063
> 302200, 302250, 2884
> 302250, 302300, 2007
> 302300, 302350, 2209
> 302350, 302400, 737
> 302400, 302450, 2398
> 302450, 302500, 2856
> 302500, 302550, 2090
> 302550, 302600, 2866
> 302600, 302625, 1786
> 302625, 302650, 1966
> 302650, 302675, 1583
> 302675, 302700, 307
> 302700, 302725, 2246
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170707/9039f750/attachment-0001.html>