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

[Magnus Ihse Bursie]

It might be the case that we should not really continue with this as 
long as we have JDK 8 as default boot jdk.

If so, we should at least add a -Xshare:auto, which will allow the user 
to manually add a system-wide CDS archive to their boot jdk, and benefit 
from it.

And then we'll revisit this when we switch to JDK 9, to see if 
"-XX:-VerifySharedSpaces -XX:SharedArchiveFile=x" is worth the effort.

Sounds reasonable?

/Magnus


On 2017-10-03 16:45, Erik Joelsson wrote:
> 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
>>>>>>
>>>
>