[webkit-dev] Switching open source Debug bots to building and testing with configuration --force-opt=O3

Fri Jun 19 13:14:02 PDT 2020

> On Jun 19, 2020, at 1:04 PM, Alexey Proskuryakov <ap at webkit.org> wrote:
> 
> 
> 
>> 19 июня 2020 г., в 10:24 AM, Geoffrey Garen <ggaren at apple.com <mailto:ggaren at apple.com>> написал(а):
>> 
>>>> On Jun 19, 2020, at 8:04 AM, Geoffrey Garen <ggaren at apple.com <mailto:ggaren at apple.com>> wrote:
>>>> 
>>>> Can you explain more about what "O3 with no-inlining” is? How does --force-opt=O3 avoid inlining? Would this fully resolve Simon concern about stack traces, or would something still be different about stack traces?
>>> There doesn't exist a way to do this now, but it'd be trivial to add a way. I won't claim it fixes all stack traces differences, but I'd think compiling using "-fno-inline -fno-optimize-sibling-calls" would get us pretty far in crashing stack traces being similar enough.
>> 
>> Sounds good.
>> 
>> I think we should try to refine the proposal along these lines, to minimize the downsides. I won’t speak for Simon, but for me, being able to ensure a clear backtrace from a bot would be a big improvement.
> 
> Enabling some level of optimization is reasonable; whether it should be -O3 with inlining disabled or -Og is a technical question that probably can't be answered without hard data. Also, building locally in the same way as bots do could be a show stopper, as people don't like slow builds.
Just to be super clear, I'm not proposing turning off inlining for JSC debug testers. We on JSC are happy with stack traces that aren't perfect.

My no inlining is a proposal for folks if we wanted this more broadly in our testing. I don't know what the results of inlining are on the time Debug+O3 takes to. run. If it's negligible, we'd also be happy to run this way for JSC Debug. But if JSC is the only thing making a change, we're going to run with inlining on.

> 
>>>> And again, I think this discussion would get a lot more focused if the change could apply only to JSC code, and not also to WTF code.
>>> I believe Mark's proposal, initially, is just to make JSC do this. So I don't see the point of compiling WTF differently. JSC can kick off its own build, and run Debug+O3 tests faster than it can run Debug+O0 tests. Given people working on JSC want this, and people working on JSC defend these tests, and that these test results are more stable (see below), we should make this change for JSC.
>>> 
>>> I was trying to convince folks defending non-JSC testing, that they too, should want this. I'm not going to pull teeth here. If folks want their tests to take ~10x longer to run, they're entitled to make that tradeoff.
>> 
>> Got it.
> 
> To do this only for JSC builds, we'd need separate builders and storage, so it becomes a question of allocating more resources, not just switching over to a different configuration. While EWS builds for JSC independently, post-commit testing shares build artifacts across all testers.
As Mark has shown, we can build locally and run faster than using a plain debug build. So as a straw man, this should just work and be faster than what we have today.

- Saam

> 
>>>> And again, on the run more tests front, it would be helpful to know whether this change was enough to get the stress tests running or not.
>>> My experience running the tests locally supports this fully. I don't get timeouts when running O3+Debug locally. When running Debug+O0 locally, I'd get timeouts all the time, and the total test run would take ~4-8 hours. We can wait for official confirmation from Mark.
>> 
>> Alexey, do the JSC stress tests run now on bots? If not, how fast would they need to run in order to be eligible to run on bots?
> 
> I don't think that there is a simple answer, as certain variations of stress tests get disabled on certain bots, JSC tests have a lot of variations that are handpicked. I wouldn't even know how to find the complex answer, but perhaps you can get the answer from <https://build.webkit.org/dashboard/ <https://build.webkit.org/dashboard/>>
> 
> - Alexey
> 
> 
> 
>> Thanks,
>> Geoff
>> 
>>> 
>>> - Saam
>>> 
>>>> 
>>>> Thanks,
>>>> Geoff
>>>> 
>>>>> On Jun 18, 2020, at 9:30 PM, Saam Barati <sbarati at apple.com <mailto:sbarati at apple.com>> wrote:
>>>>> 
>>>>> Why are we insisting on doing something on the bots that takes ~10x longer to run than necessary? I’d rather have that time spent running more tests.
>>>>> 
>>>>> Overall, how we’re doing things now feels like a bad allocation of bot resources. The differences I see between O3 with no-inlining vs O0 is:
>>>>> - Some race conditions will behave differently. Race conditions are already non predictable. I don’t think we’re losing anything here.
>>>>> - O0 vs O3 is a different compiler. We may encounter bugs in O3 we don’t in O0, and vice versa. In general, we probably care more about O3 compiler bugs than O0, since we don’t ship O0, but ship a lot of O3.
>>>>> 
>>>>> (And if we’re going to insist on “I want it to run what I build at my desk”: I run debug with O3 at my desk, and I can run debug tests in a reasonable amount of time now.)
>>>>> 
>>>>> In evaluating what’s the better setup, I think it’s helpful to think about this from the other side. Let’s imagine we had Debug+O3 as our current setup. And someone proposed to move it to O0 and make our tests take ~10x longer. I think that’d be a non-starter.
>>>>> 
>>>>> - Saam
>>>>> 
>>>>>> On Jun 17, 2020, at 9:48 PM, Simon Fraser <simon.fraser at apple.com <mailto:simon.fraser at apple.com>> wrote:
>>>>>> 
>>>>>> I also object to losing good stack traces for crashes on Debug bots.
>>>>>> 
>>>>>> Also, I don't think Debug bots should build something different from what I build at my desk.
>>>>>> 
>>>>>> Simon
>>>>>> 
>>>>>>> On Jun 17, 2020, at 1:36 PM, Mark Lam <mark.lam at apple.com <mailto:mark.lam at apple.com>> wrote:
>>>>>>> 
>>>>>>> Hi folks,
>>>>>>> 
>>>>>>> We're planning to switch the JSC EWS bot and build.webkit.org <http://build.webkit.org/> Debug build and test bots to building with the following set first:
>>>>>>> ./Tools/Scripts/set-webkit-configuration --force-opt=O3
>>>>>>> 
>>>>>>> This means the Debug builds will be built with optimization level forced to O3.
>>>>>>> 
>>>>>>> Why are we doing this?
>>>>>>> 1. So that the JSC EWS will start catching ASSERT failures.
>>>>>>> 2. JSC stress test Debug bots have been timing out and not running tests at all.  Hopefully, this change will fix this issue.
>>>>>>> 3. Tests will run to completion faster and we’ll catch regressions sooner.
>>>>>>> 
>>>>>>> The downside: crash stack traces will be like Release build stack traces.  But I don’t think we should let this deter us.  It’s not like there’s no stack information.  And just as we do with debugging Release build test failures, we can always do a Debug build locally to do our debugging.
>>>>>>> 
>>>>>>> We would like to apply this change to all Debug build and test bots, not just the JSC ones.  Does anyone strongly object to this change?
>>>>>>> 
>>>>>>> Thanks.
>>>>>>> 
>>>>>>> Cheers,
>>>>>>> Mark
>>>>>>> 
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