[lld] r194545 - [PECOFF] Fix use-after-return.

Wed Nov 13 16:19:13 PST 2013

On Wed, Nov 13, 2013 at 2:41 PM, Rick Foos <rfoos at codeaurora.org> wrote:

>  Sorry for the delay,
>
> Our problem with running the sanitizers is that the load average running
> under Ninja reached 146 and a short time after a system crash requiring
> calling someone to power cycle the box...
>

I'm curious what is causing so much load? All our tests are mostly
single-threaded, so if only #cores jobs are spawned (or #cores + 2 which is
what ninja uses when #cores > 2), there should only be #cores + 2 jobs
running simultaneously (certainly not 146/32 ~4.5). Is lit spawning too
many jobs?

Does the machine have enough RAM?

>
> The address sanitizer by itself leaves a load average 40. This means the
> OS over 100% utilization, and is thrashing a bit. Load Average doesn't say
> what exactly is thrashing.
>
> Ninja supports make's -j, and -l options. The -l maximum load average, is
> the key.
>
> The load average should be less than the total number of cores
> (hyperthreads too) before Ninja launches another task.
>
> A Load Average at or lower than 100%  technically should benefit
> performance, and maximize throughput. However, I will be happy if I don't
> have to call someone to power cycle the server :)
>

I don't think that's quite how it works. As long as you have enough RAM,
the only performance loss due to having a bunch of jobs waiting is context
switching overhead, but that can be minimized by either lowering the
preempt timer rate (what is called HZ in linux; 100 which is common for
servers doing batch jobs dilutes the overhead to basically nothing) or if
you are running a recent kernel then you can arrange things to run tickless
and then there will be essentially no overhead. If load is less than
#cores, then you don't have a job running on every core, which means that
those cores are essentially idle and you are losing performance. The other
killer is jobs blocking on disk IO *with no other jobs to be scheduled in
the meantime*; generally you have to keep load above 100% to avoid that
problem.

-- Sean Silva

>
> So the maximum load average of a 16 core machine with hyperthreads is 32
> (keeping it simple). This needs to be passed to all make's and Ninja build
> steps on that slave to maximize throughput.
>
> For now, I'm looking at a minimal patch to include jobs and a new
> loadaverage variable for the sanitizers.
>
> Longer term, all buildslaves should define maximum loadaverage, and all
> make/ninja steps should pass -j, and -l options.
>
> Best Regards,
> Rick
>
>
> On 11/13/2013 11:21 AM, Sergey Matveev wrote:
>
> +kcc
>
>
> On Wed, Nov 13, 2013 at 6:41 AM, Shankar Easwaran <shankare at codeaurora.org
> > wrote:
>
>> Sorry for another indirection. Rick foos is working on it. I think there
>> is some good news here :)
>>
>> Cced Rick + adding Galina,Dmitri.
>>
>> Thanks
>>
>> Shankar Easwaran
>>
>>
>> On 11/12/2013 8:37 PM, Rui Ueyama wrote:
>>
>>> Shankar tried to set it up recently.
>>>
>>>
>>> On Tue, Nov 12, 2013 at 6:31 PM, Sean Silva <silvas at purdue.edu> wrote:
>>>
>>>  Sanitizers?
>>>>
>>>> There have been a couple of these sorts of bugs recently... we really
>>>> ought to have some sanitizer bots...
>>>>
>>>> -- Sean Silva
>>>>
>>>>
>>>> On Tue, Nov 12, 2013 at 9:21 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>>
>>>>  Author: ruiu
>>>>> Date: Tue Nov 12 20:21:51 2013
>>>>> New Revision: 194545
>>>>>
>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=194545&view=rev
>>>>> Log:
>>>>> [PECOFF] Fix use-after-return.
>>>>>
>>>>> Modified:
>>>>>      lld/trunk/lib/Driver/WinLinkDriver.cpp
>>>>>
>>>>> Modified: lld/trunk/lib/Driver/WinLinkDriver.cpp
>>>>> URL:
>>>>>
>>>>> http://llvm.org/viewvc/llvm-project/lld/trunk/lib/Driver/WinLinkDriver.cpp?rev=194545&r1=194544&r2=194545&view=diff
>>>>>
>>>>>
>>>>> ==============================================================================
>>>>> --- lld/trunk/lib/Driver/WinLinkDriver.cpp (original)
>>>>> +++ lld/trunk/lib/Driver/WinLinkDriver.cpp Tue Nov 12 20:21:51 2013
>>>>> @@ -842,7 +842,7 @@ WinLinkDriver::parse(int argc, const cha
>>>>>
>>>>>       case OPT_INPUT:
>>>>>         inputElements.push_back(std::unique_ptr<InputElement>(
>>>>> -          new PECOFFFileNode(ctx, inputArg->getValue())));
>>>>> +          new PECOFFFileNode(ctx,
>>>>> ctx.allocateString(inputArg->getValue()))));
>>>>>         break;
>>>>>
>>>>>   #define DEFINE_BOOLEAN_FLAG(name, setter)       \
>>>>> @@ -892,9 +892,11 @@ WinLinkDriver::parse(int argc, const cha
>>>>>     // start with a hypen or a slash. This is not compatible with
>>>>> link.exe
>>>>>     // but useful for us to test lld on Unix.
>>>>>     if (llvm::opt::Arg *dashdash =
>>>>> parsedArgs->getLastArg(OPT_DASH_DASH)) {
>>>>> -    for (const StringRef value : dashdash->getValues())
>>>>> -      inputElements.push_back(
>>>>> -          std::unique_ptr<InputElement>(new PECOFFFileNode(ctx,
>>>>> value)));
>>>>> +    for (const StringRef value : dashdash->getValues()) {
>>>>> +      std::unique_ptr<InputElement> elem(
>>>>> +          new PECOFFFileNode(ctx, ctx.allocateString(value)));
>>>>> +      inputElements.push_back(std::move(elem));
>>>>> +    }
>>>>>     }
>>>>>
>>>>>     // Add the libraries specified by /defaultlib unless they are
>>>>> already
>>>>> added
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> llvm-commits mailing list
>>>>> llvm-commits at cs.uiuc.edu
>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>>>>
>>>>>
>>>>
>>
>>   --
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted
>> by the Linux Foundation
>>
>>
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>
>
>
>
> _______________________________________________
> llvm-commits mailing listllvm-commits at cs.uiuc.eduhttp://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
>
>
> --
> Rick Foos
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation
>
>
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20131113/3171df5d/attachment.html>