RFR: JDK-8188312 Use CDS if present when running the Boot JDK during build

[Erik Joelsson]

The change looks good, but unfortunately I don't see a lot of gain. 
First of all it seems the -XX:SharedArchiveFile option removes a lot of 
the performance gain. Here are some rough numbers from my machine. All 
of them are "time make images" with a clean build directory, on a 16 
core/32 threads linux box. In some cases I did two runs, which further 
shows the amount of variance in the build time.

Baseline: boot jdk 8, no cds
real    3m14.688s
user    38m34.132s
sys    5m4.032s

real    3m11.086s
user    38m16.820s
sys    4m35.344s

Magnus second patch: boot jdk 8, cds SharedArchiveFile
real    3m6.268s
user    38m9.168s
sys    4m13.392s

real    3m15.985s
user    37m55.328s
sys    3m50.140s

Magnus first patch: boot jdk 8, cds in classes.jsa
real    2m55.972s
user    37m59.636s
sys    4m1.020s

Baseline with boot jdk 9, no cds
real    3m14.262s
user    40m9.216s
sys    5m5.480s

Magnus first patch: boot jdk 9, cds in classes.jsa
real    3m12.721s
user    39m29.332s
sys    4m25.472s

Magnus second patch: boot jdk 9, -XX:SharedArchiveFile, 
-XX:-VerifySharedSpaces
real    3m4.297s
user    39m24.556s
sys    4m31.980s

So from this (certainly limited) data set, the only configuration that 
shows meaningful improvement is with boot jdk 8 and the first patch, 
where we stealth add classes.jsa to the boot jdk. I find this a very bad 
thing to do in principle so would vote against such a solution anyway. 
Also note that we will switch default boot from 8 to 9 very soon, so 
it's really the results on 9 that are relevant here. In 9, there is 
another option, -XX:-VerifySharedSpaces which removes most of the 
overhead added by -XX:SharedArchiveFile. Perhaps using that this becomes 
worth while.

I would like to see more data showing the optimization is actually 
relevant. On my machine it seems to make a measurable difference, but 
not a very significant one.

One note on the proposed change. While working on this, when I changed 
my boot jdk, configure did not regenerate the classes.jsa file. I 
believe for safety we should regenerate on every configure run. It's not 
that expensive anyway.

/Erik


On 2017-10-03 14:55, David Holmes wrote:
> Looks good to me.
>
> Thanks,
> David
>
> On 3/10/2017 10:47 PM, Magnus Ihse Bursie wrote:
>> On 2017-10-03 14:21, David Holmes wrote:
>>> Erik J. raises a good point in the bug report that 
>>> -XX:SharedArchiveFile=xxx should be used if we create the archive. 
>>> The build system has no business creating an archive inside the boot 
>>> JDK.
>>
>> Agree, that is a better solution. I was not aware of the 
>> -XX:SharedArchiveFile option.
>>
>> Here's an updated webrev:
>> http://cr.openjdk.java.net/~ihse/JDK-8188312-use-CDS-for-bootjdk/webrev.02 
>>
>>
>> I create the jsa file in configure-support, that way it can survive a 
>> "make clean".
>>
>> /Magnus
>>
>>>
>>> David
>>>
>>> On 3/10/2017 9:02 PM, David Holmes wrote:
>>>> Hi Claes,
>>>>
>>>> On 3/10/2017 8:48 PM, Claes Redestad wrote:
>>>>> Hi,
>>>>>
>>>>> -Xshare:auto silently ignores failures to map the shared archive 
>>>>> and should be safe to use. I think you're thinking of -Xshare:on 
>>>>> which will fail/abort the VM if mapping fails.
>>>>
>>>> Ah okay.
>>>>
>>>> In that case seems reasonable. But please test thoroughly across 
>>>> all platforms in JPRT.
>>>>
>>>> Thanks,
>>>> David
>>>>
>>>>> /Claes
>>>>>
>>>>>
>>>>> On 2017-10-03 12:28, David Holmes wrote:
>>>>>> Hi Magnus,
>>>>>>
>>>>>> As I just put in the bug report, it isn't quite this simple. You 
>>>>>> have to be able to tolerate/recover from failure to map the 
>>>>>> shared archive.
>>>>>>
>>>>>> Cheers,
>>>>>> David
>>>>>>
>>>>>> On 3/10/2017 8:24 PM, Magnus Ihse Bursie wrote:
>>>>>>> We should use CDS to minimize Java startup time during build. We 
>>>>>>> run multiple Java commands, and every second counts.
>>>>>>>
>>>>>>> On my machine, I get a ~3% build time speedup with this fix.
>>>>>>>
>>>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8188312
>>>>>>> WebRev: 
>>>>>>> http://cr.openjdk.java.net/~ihse/JDK-8188312-use-CDS-for-bootjdk/webrev.01 
>>>>>>>
>>>>>>>
>>>>>>> /Magnus
>>>>>
>>