debug in hotspot

[Krystal Mok]

On Fri, May 4, 2012 at 8:48 AM, Xin Tong <xerox.time.tech at gmail.com> wrote:

> On Thu, May 3, 2012 at 8:45 PM, Xin Tong <xerox.time.tech at gmail.com>
> wrote:
> > On Thu, May 3, 2012 at 8:29 PM, Krystal Mok <rednaxelafx at gmail.com>
> wrote:
> >> On 2012-5-4, at 8:16, Xin Tong <xerox.time.tech at gmail.com> wrote:
> >>
> >>> i would like to know how to debug in hotspot, what are some of the
> >>> standard procedures ? Lets say, the hotspot fails to run a Java
> >>> program. a few steps to triage the problem.
> >>>
> >>> 1. disable JIT compilers. hotspot has 2 compilers c1 and c2. they are
> >>> both used if TieredCompilation is enabled. How do I disable both of
> >>> them.
> >>
> >> It's there in the globals.hpp file: -XX:-UseCompiler
>

I forgot to mention there's a shorter (and better supported) equivalent to
this: -Xint


> >>
> >>> 2. if the Java program passes after the JIT is disabled. There is a
> >>> good chance that the problem is cause by the JIT. I want to narrow
> >>> down to a specific method, I can use PrintCompilation to find out all
> >>> the method compiled, but how can I disable one specific method from
> >>> being compiled.
> >>
> >> -XX:CompileCommand='exclude,your/class,yourMethod'
> >> Or an equivalent .hotspot_compiler configuration file.
> >> Read this thread for example (in Chinese):
> http://hllvm.group.iteye.com/group/topic/28201
>
> By the way, one thing we can do here is to force c1 to compile a
> method. i.e. c2 is more likely to fail than c1 as it does more
> optimizations. how do i force c1 to compile a method ?


What platform and version of JDK are you using?
If there's a Client VM for your platform, use that;
If you're using JDK7 or JDK8, try -XX:+TieredCompilation
-XX:TieredStopAtLevel=1, which effectively disables C2 in tiered mode.
Otherwise, if you have to use the Server VM (for instance, on amd64 where
Oracle doesn't provide a Client VM build), and you're using an old version
of JDK6, I'm not sure if there's a way to force compilation with C1.
And, there's no way (yet) to force compilation with C1/C2 on a
per-method granularity.


> and how do i
> know which compiler is used to compile a method in the
> PrintCompilation log  or through any other options ?
>

Please refer to this note: https://gist.github.com/1165804#file_notes.md
I was supposed to put this up in the HotSpotInternals wiki, but somehow I
haven't done it yet...oops.

- Kris


>
> >>
> >>> 3. if i am able to limit down to a specific method, i want to start
> >>> disabling optimizations, is there an option to disable all
> >>> optimizations in the c1 and c2 compilers, or i need to do it manually
> >>> ?
> >>>
> >>
> >> I don't think there's an all-in-one flag to turn all optimizations off.
> At least not for now.
> >> See c1_globals.hpp and c2_globals.hpp to get an idea of what
> optimizations you can turn off.
> >>
> >> - Kris
> >>
> >>> Thanks
> >
> > Thank you very much Kris. This is very helpful.
>
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