Stop using precompiled headers for Linux?

Magnus Ihse Bursie magnus.ihse.bursie at
Fri Nov 2 11:14:17 UTC 2018

On 2018-11-02 11:39, Magnus Ihse Bursie wrote:
> On 2018-11-02 00:53, Ioi Lam wrote:
>> Maybe precompiled.hpp can be periodically (weekly?) updated by a 
>> robot, which parses the dependencies files generated by gcc, and pick 
>> the most popular N files?
> I think that's tricky to implement automatically. However, I've done 
> more or less, that, and I've got some wonderful results! :-)

Ok, I'm done running my tests.

TL;DR: I've managed to reduce wall-clock time from 2m 45s (with pch) or 
2m 23s (without pch), to 1m 55s. The cpu time spent went from 52m 27s 
(with pch) or 55m 30s (without pch) to 41m 10s. This is a huge gain for 
our automated builds! And a clear improvement even for the ordinary 

The list of included header files is reduced to just 37. The winning 
combination was to include all header files that was included in more 
than 130 different files, but to exclude all files with the name 
"*.inline.hpp". Hopefully, a further gain of not pulling in the 
*.inline.hpp files is that the risk of pch/non-pch failures will diminish.

However, these 37 files in turn pull in an additional 201 header files. 
Of these, three are *.inline.hpp:
os_cpu/linux_x86/bytes_linux_x86.inline.hpp and 
os_cpu/linux_x86/copy_linux_x86.inline.hpp. This looks like a problem 
with the header files to me.

With some exceptions (mostly related to JFR), these additional 200 files 
have "generic" looking names (like share/gc/g1/g1_globals.hpp), which 
indicate to me that it is reasonable to have them in this list, just as 
the list of the original 37 tended to be quite general and high-level 
includes. However, some files (like 
share/jfr/instrumentation/jfrEventClassTransformer.hpp) has maybe leaked 
in where they should not really be. It might be worth letting a hotspot 
engineer spend some cycles to check up these files and see if anything 
can be improved.

Caveats: I have only run this on my local linux build with the default 
server JVM configuration. Other machines will have different sweet 
spots. Other JVM variants/feature combinations will have different sweet 
spots. And, most importantly, I have not tested this at all on Windows. 
Nevertheless, I'm almost prepared to suggest a patch that uses this 
selection of files if running on gcc, just as is, because of the speed 
improvements I measured.

And some data:

Here is my log from my runs. The "on or above" means the cutoff I used 
for how many files that needed to include the files that were selected. 
As you can see, there is not much difference between cutoffs between 
130-150, or (without the inline files) between 110 and 150. (There were 
a lot of additional inline files in the positions below 130.) With all 
other equal, I'd prefer a solution with fewer files. That is less likely 
to go bad.

real    2m45.623s
user    52m27.813s
sys    5m27.176s
hotspot with original pch

real    2m23.837s
user    55m30.448s
sys    3m39.739s
hotspot without pch

real    1m59.533s
user    42m50.019s
sys    3m0.893s
hotspot new pch on or above 250

real    1m58.937s
user    42m18.994s
sys    3m0.245s
hotspot new pch on or above 200

real    2m0.729s
user    42m16.636s
sys    2m57.125s
hotspot new pch on or above 170

real    1m58.064s
user    42m9.618s
sys    2m57.635s
hotspot new pch on or above 150

real    1m58.053s
user    42m9.796s
sys    2m58.732s
hotspot new pch on or above 130

real    2m3.364s
user    42m54.818s
sys    3m2.737s
hotspot new pch on or above 100

real    2m6.698s
user    44m30.434s
sys    3m12.015s
hotspot new pch on or above 70

real    2m0.598s
user    41m17.810s
sys    2m56.258s
hotspot new pch on or above 150 without inline

real    1m55.981s
user    41m10.076s
sys    2m51.983s
hotspot new pch on or above 130 without inline

real    1m56.449s
user    41m10.667s
sys    2m53.808s
hotspot new pch on or above 110 without inline

And here is the "winning" list (which I declared as "on or above 130, 
without inline"). I encourage everyone to try this on their own system, 
and report back the results!

# include "classfile/classLoaderData.hpp"
# include "classfile/javaClasses.hpp"
# include "classfile/systemDictionary.hpp"
# include "gc/shared/collectedHeap.hpp"
# include "gc/shared/gcCause.hpp"
# include "logging/log.hpp"
# include "memory/allocation.hpp"
# include "memory/iterator.hpp"
# include "memory/memRegion.hpp"
# include "memory/resourceArea.hpp"
# include "memory/universe.hpp"
# include "oops/instanceKlass.hpp"
# include "oops/klass.hpp"
# include "oops/method.hpp"
# include "oops/objArrayKlass.hpp"
# include "oops/objArrayOop.hpp"
# include "oops/oop.hpp"
# include "oops/oopsHierarchy.hpp"
# include "runtime/atomic.hpp"
# include "runtime/globals.hpp"
# include "runtime/handles.hpp"
# include "runtime/mutex.hpp"
# include "runtime/orderAccess.hpp"
# include "runtime/os.hpp"
# include "runtime/thread.hpp"
# include "runtime/timer.hpp"
# include "services/memTracker.hpp"
# include "utilities/align.hpp"
# include "utilities/bitMap.hpp"
# include "utilities/copy.hpp"
# include "utilities/debug.hpp"
# include "utilities/exceptions.hpp"
# include "utilities/globalDefinitions.hpp"
# include "utilities/growableArray.hpp"
# include "utilities/macros.hpp"
# include "utilities/ostream.hpp"
# include "utilities/ticks.hpp"


> I'd still like to run some more tests, but preliminiary data indicates 
> that there is much to be gained by having a more sensible list of 
> files in the precompiled header.
> The fewer files we got on this list, the less likely it is to become 
> (drastically) outdated. So I don't think we need to do this 
> automatically, but perhaps manually every now and then when we feel 
> build times are increasing.
> /Magnus
>> - Ioi
>> On 11/1/18 4:38 PM, David Holmes wrote:
>>> It's not at all obvious to me that the way we use PCH is the 
>>> right/best way to use it. We dump every header we think it would be 
>>> good to precompile into precompiled.hpp and then only ask gcc to 
>>> precompile it. That results in a ~250MB file that has to be read 
>>> into and processed for every source file! That doesn't seem very 
>>> efficient to me.
>>> Cheers,
>>> David
>>> On 2/11/2018 3:18 AM, Erik Joelsson wrote:
>>>> 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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