building libjvm with -Os for space optimization - was : RE: RFR: 8234525: enable link-time section-gc for linux s390x to remove unused code

[Ioi Lam]

On 11/27/19 10:03 AM, Doerr, Martin wrote:
> Hi Claes,
>
> that kind of surprises me. I'd expect files which rather benefit from -O3 to be far less than those which benefit from -Os.
> Most performance critical code lives inside the code cache and is not dependent on C++ compiler optimizations.
> I'd expect GC code, C2's register allocation and a few runtime files to be the most performance critical C++ code.
> So the list of files for -Os may become long.

Class loading/verification/resolution are also sensitive to C++ speed.

Thanks
- Ioi

> Yeah, I think we should use native profiling information to find out what's really going on.
>
> Your idea to change file by file and check for performance regression makes sense to me, though.
>
> Best regards,
> Martin
>
>
>> -----Original Message-----
>> From: Claes Redestad <claes.redestad at oracle.com>
>> Sent: Mittwoch, 27. November 2019 18:57
>> To: Baesken, Matthias <matthias.baesken at sap.com>; Doerr, Martin
>> <martin.doerr at sap.com>; Erik Joelsson <erik.joelsson at oracle.com>; 'build-
>> dev at openjdk.java.net' <build-dev at openjdk.java.net>; 'hotspot-
>> dev at openjdk.java.net' <hotspot-dev at openjdk.java.net>
>> Subject: Re: building libjvm with -Os for space optimization - was : RE: RFR:
>> 8234525: enable link-time section-gc for linux s390x to remove unused code
>>
>> Hi,
>>
>> we discussed doing the opposite for Mac OS X recently, where builds are
>> currently set to -Os by default. -O3 helped various networking
>> (micro)benchmarks by up to 20%.
>>
>> Rather than doing -Os by default and then cherry-pick things over to -O3
>> on a case-by-case basis, I'd suggest the opposite: keep -O3 as the
>> default, start evaluating -Os on a case-by-case basis. This allows for
>> an incremental approach where we identify things that are definitely not
>> performance critical, e.g., never shows up in profiles, and switch those
>> compilation units over to -Os. Check for harmful performance impact and
>> expected footprint improvement; rinse; repeat.
>>
>> $.02
>>
>> /Claes
>>
>>
>> On 2019-11-27 17:36, Baesken, Matthias wrote:
>>> Hello Martin,  I checked  building  libjvm.so  with -Os  (instead of -O3) .
>>>
>>> I used gcc-7  on linux x86_64 .
>>> The size  of  libjvm.so   dropped   from    24M  (normal night make with -O3)
>> to   18M   ( test make with -Os)   .
>>>    (adding the link-time gc might  reduce the size by another ~ 10 % ,  but
>> those 2 builds were without the ltgc )
>>> Cannot say much so far about performance impact .
>>>
>>> Best regards, Matthias
>>>
>>>
>>>
>>>> Hi Matthias and Erik,
>>>>
>>>> I also think this is an interesting option.
>>>>
>>>> I like the idea to generate smaller libraries. In addition to that, I could also
>>>> imagine building with -Os (size optimized) by default and only select -O3
>> for
>>>> performance critical files (e.g. C2's register allocation, some gc code, ...).
>>>>
>>>> If we want to go into such a direction for all linux platforms and want to
>> use
>>>> this s390 only change as some kind of pipe cleaner, I think this change is
>> fine
>>>> and can get pushed.
>>>> Otherwise, I think building s390 differently and not intending to do the
>> same
>>>> for other linux platforms would be not so good.
>>>>
>>>> We should only make sure the exported symbols are set up properly to
>> avoid
>>>> that this optimization throws out too much.
>>>>
>>>> My 50 Cents.
>>>>
>>>> Best regards,
>>>> Martin
>>>>