Stop using precompiled headers for Linux?

Erik Joelsson erik.joelsson at oracle.com
Thu Nov 1 17:18:22 UTC 2018


Hello,

My point here, which wasn't very clear, is that Mac and Linux seem to 
lose just as much real compile time. The big difference in these tests 
was rather the number of cpus in the machine (32 threads in the linux 
box vs 8 on the mac). The total amount of work done was increased when 
PCH was disabled, that's the user time. Here is my theory on why the 
real (wall clock) time was not consistent with user time between these 
experiments can be explained:

With pch the time line (simplified) looks like this:

1. Single thread creating PCH
2. All cores compiling C++ files

When disabling pch it's just:

1. All cores compiling C++ files

To gain speed with PCH, the time spent in 1 much be less than the time 
saved in 2. The potential time saved in 2 goes down as the number of 
cpus go up. I'm pretty sure that if I repeated the experiment on Linux 
on a smaller box (typically one we use in CI), the results would look 
similar to Macosx, and similarly, if I had access to a much bigger mac, 
it would behave like the big Linux box. This is why I'm saying this 
should be done for both or none of these platforms.

In addition to this, the experiment only built hotspot. If you we would 
instead build the whole JDK, then the time wasted in 1 in the PCH case 
would be negated to a large extent by other build targets running 
concurrently, so for a full build, PCH is still providing value.

The question here is that if the value of PCH isn't very big, perhaps 
it's not worth it if it's also creating as much grief as described here. 
There is no doubt that there is value however. And given the examination 
done by Magnus, it seems this value could be increased.

The main reason why we haven't disabled PCH in CI before this. We really 
really want to get CI builds fast. We don't have a ton of over capacity 
to just throw at it. PCH made builds faster, so we used them. My other 
reason is consistency between builds. Supporting multiple different 
modes of building creates the potential for inconsistencies. For that 
reason I would definitely not support having PCH on by default, but 
turned off in our CI/dev-submit. We pick one or the other as the 
official build configuration, and we stick with the official build 
configuration for all builds of any official capacity (which includes CI).

In the current CI setup, we have a bunch of tiers that execute one after 
the other. The jdk-submit currently only runs tier1. In tier2 I've put 
slowdebug builds with PCH disabled, just to help verify a common 
developer configuration. These builds are not meant to be used for 
testing or anything like that, they are just run for verification, which 
is why this is ok. We could argue that it would make sense to move the 
linux-x64-slowdebug without pch build to tier1 so that it's included in 
dev-submit.

/Erik

On 2018-11-01 03:38, Magnus Ihse Bursie wrote:
>
>
> On 2018-10-31 00:54, Erik Joelsson wrote:
>> Below are the corresponding numbers from a Mac, (Mac Pro (Late 2013), 
>> 3.7 GHz, Quad-Core Intel Xeon E5, 16 GB). To be clear, the -npch is 
>> without precompiled headers. Here we see a slight degradation when 
>> disabling on both user time and wall clock time. My guess is that the 
>> user time increase is about the same, but because of a lower cpu 
>> count, the extra load is not as easily covered.
>>
>> These tests were run with just building hotspot. This means that the 
>> precompiled header is generated alone on one core while nothing else 
>> is happening, which would explain this degradation in build speed. If 
>> we were instead building the whole product, we would see a better 
>> correlation between user and real time.
>>
>> Given the very small benefit here, it could make sense to disable 
>> precompiled headers by default for Linux and Mac, just as we did with 
>> ccache.
>>
>> I do know that the benefit is huge on Windows though, so we cannot 
>> remove the feature completely. Any other comments?
>
> Well, if you show that it is a loss in time on macosx to disable 
> precompiled headers, and no-one (as far as I've seen) has complained 
> about PCH on mac, then why not keep them on as default there? That the 
> gain is small is no argument to lose it. (I remember a time when you 
> were hunting seconds in the build time ;-))
>
> On linux, the story seems different, though. People experience PCH as 
> a problem, and there is a net loss of time, at least on selected 
> testing machines. It makes sense to turn it off as default, then.
>
> /Magnus
>
>>
>> /Erik
>>
>> macosx-x64
>> real     4m13.658s
>> user     27m17.595s
>> sys     2m11.306s
>>
>> macosx-x64-npch
>> real     4m27.823s
>> user     30m0.434s
>> sys     2m18.669s
>>
>> macosx-x64-debug
>> real     5m21.032s
>> user     35m57.347s
>> sys     2m20.588s
>>
>> macosx-x64-debug-npch
>> real     5m33.728s
>> user     38m10.311s
>> sys     2m27.587s
>>
>> macosx-x64-slowdebug
>> real     3m54.439s
>> user     25m32.197s
>> sys     2m8.750s
>>
>> macosx-x64-slowdebug-npch
>> real     4m11.987s
>> user     27m59.857s
>> sys     2m18.093s
>>
>>
>> On 2018-10-30 14:00, Erik Joelsson wrote:
>>> Hello,
>>>
>>> On 2018-10-30 13:17, Aleksey Shipilev wrote:
>>>> On 10/30/2018 06:26 PM, Ioi Lam wrote:
>>>>> Is there any advantage of using precompiled headers on Linux?
>>>> I have measured it recently on shenandoah repositories, and 
>>>> fastdebug/release build times have not
>>>> improved with or without PCH. Actually, it gets worse when you 
>>>> touch a single header that is in PCH
>>>> list, and you end up recompiling the entire Hotspot. I would be in 
>>>> favor of disabling it by default.
>>> I just did a measurement on my local workstation (2x8 cores x2 ht 
>>> Ubuntu 18.04 using Oracle devkit GCC 7.3.0). I ran "time make 
>>> hotspot" with clean build directories.
>>>
>>> linux-x64:
>>> real    4m6.657s
>>> user    61m23.090s
>>> sys    6m24.477s
>>>
>>> linux-x64-npch
>>> real    3m41.130s
>>> user    66m11.824s
>>> sys    4m19.224s
>>>
>>> linux-x64-debug
>>> real    4m47.117s
>>> user    75m53.740s
>>> sys    8m21.408s
>>>
>>> linux-x64-debug-npch
>>> real    4m42.877s
>>> user    84m30.764s
>>> sys    4m54.666s
>>>
>>> linux-x64-slowdebug
>>> real    3m54.564s
>>> user    44m2.828s
>>> sys    6m22.785s
>>>
>>> linux-x64-slowdebug-npch
>>> real    3m23.092s
>>> user    55m3.142s
>>> sys    4m10.172s
>>>
>>> These numbers support your claim. Wall clock time is actually 
>>> increased with PCH enabled, but total user time is decreased. Does 
>>> not seem worth it to me.
>>>>> It's on by default and we keep having
>>>>> breakage where someone would forget to add #include. The latest 
>>>>> instance is JDK-8213148.
>>>> Yes, we catch most of these breakages in CIs. Which tells me adding 
>>>> it to jdk-submit would cover
>>>> most of the breakage during pre-integration testing.
>>> jdk-submit is currently running what we call "tier1". We do have 
>>> builds of Linux slowdebug with precompiled headers disabled in 
>>> tier2. We also build solaris-sparcv9 in tier1 which does not support 
>>> precompiled headers at all, so to not be caught in jdk-submit you 
>>> would have to be in Linux specific code. The example bug does not 
>>> seem to be that. Mach5/jdk-submit was down over the weekend and 
>>> yesterday so my suspicion is the offending code in this case was 
>>> never tested.
>>>
>>> That said, given that we get practically no benefit from PCH on 
>>> Linux/GCC, we should probably just turn it off by default for Linux 
>>> and/or GCC. I think we need to investigate Macos as well here.
>>>
>>> /Erik
>>>> -Aleksey
>>>>
>>>
>>
>



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