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
