[llvm-dev] llvm and clang are getting slower

[Mehdi Amini via llvm-dev]

> On Mar 8, 2016, at 1:09 PM, Sean Silva via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
> 
> On Tue, Mar 8, 2016 at 10:42 AM, Richard Smith via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> On Tue, Mar 8, 2016 at 8:13 AM, Rafael Espíndola
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> > I have just benchmarked building trunk llvm and clang in Debug,
> > Release and LTO modes (see the attached scrip for the cmake lines).
> >
> > The compilers used were clang 3.5, 3.6, 3.7, 3.8 and trunk. In all
> > cases I used the system libgcc and libstdc++.
> >
> > For release builds there is a monotonic increase in each version. From
> > 163 minutes with 3.5 to 212 minutes with trunk. For comparison, gcc
> > 5.3.2 takes 205 minutes.
> >
> > Debug and LTO show an improvement in 3.7, but have regressed again in 3.8.
> 
> I'm curious how these times divide across Clang and various parts of
> LLVM; rerunning with -ftime-report and summing the numbers across all
> compiles could be interesting.
> 
> Based on the results I posted upthread about the relative time spend in the backend for debug vs release, we can estimate this.
> To summarize:
> 10% of time spent in LLVM for Debug
> 33% of time spent in LLVM for Release
> (I'll abbreviate "in LLVM" as just "backend"; this is "backend" from clang's perspective)
> 
> Let's look at the difference between 3.5 and trunk.
> 
> For debug, the user time jumps from 174m50.251s to 197m9.932s.
> That's {10490.3, 11829.9} seconds, respectively.
> For release, the corresponding numbers are:
> {9826.71, 12714.3} seconds.
> 
> debug35 = 10490.251
> debugTrunk = 11829.932
> 
> debugTrunk/debug35 == 1.12771
> debugRatio = 1.12771
> 
> release35 = 9826.705
> releaseTrunk = 12714.288
> 
> releaseTrunk/release35 == 1.29385
> releaseRatio = 1.29385
> 
> For simplicity, let's use a simple linear model for the distribution of slowdown between the frontend and backend: a constant factor slowdown for the backend, and an independent constant factor slowdown for the frontend. This gives the following linear system:
> debugRatio = .1 * backendRatio + (1 - .1) * frontendRatio
> releaseRatio = .33 * backendRatio + (1 - .33) * frontendRatio
> 
> Solving this linear system we find that under this simple model, the expected slowdown factors are:
> backendRatio = 1.77783
> frontendRatio = 1.05547
> 
> Intuitively, backendRatio comes out larger in this comparison because we see the biggest slowdown during release (1.29 vs 1.12), and during release we are spending a larger fraction of time in the backend (33% vs 10%).
> 
> Applying this same model to across Rafael's data, we find the following (numbers have been rounded for clarity):
> 
> transition       backendRatio   frontendRatio
> 3.5->3.6         1.08           1.03
> 3.6->3.7         1.30           0.95
> 3.7->3.8         1.34           1.07
> 3.8->trunk       0.98           1.02                
> 
> Note that in Rafael's measurements LTO is pretty similar to Release from a CPU time (user time) standpoint. While the final LTO link takes a large amount of real time, it is single threaded. Based on the real time numbers the LTO link was only spending about 20 minutes single-threaded (i.e. about 20 minutes CPU time), which is pretty small compared to the 300-400 minutes of total CPU time. It would be interesting to see the numbers for -O0 or -O1 per-TU together with LTO.


Just a note about LTO being sequential: Rafael mentioned he was "building trunk llvm and clang". By default I believe it is ~56 link targets that can be run in parallel (provided you have enough RAM to avoid swapping).

-- 
Mehdi

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