fastdebug vs jvmg

[Volker Simonis]

Hi,

I can confirm that fastdebug is at least an order of magnitude faster
than debug and as such it is the only practicable way to do automatic
testing with assertions turned on. On the other hand, on most
platforms you really need a true debug build to get a decent debugging
experience.

So we daily build product and fastdebug configurations by default for
automatic testing but developers usually build a true debug (jvmg)
build for their daily work.

Regards,
Volker


On Thu, Dec 6, 2012 at 5:57 AM, David Holmes <david.holmes at oracle.com> wrote:
> First note that Jesper's email is from Oct 31 and only just turned up!
> Either that or he has a clock issue on his system :)
>
> On 6/12/2012 8:25 AM, Kelly O'Hair wrote:
>>
>> On Oct 31, 2012, at 5:26 AM, Jesper Wilhelmsson wrote:
>>>
>>> Then I reconfigured with --enable-debug.
>>>
>>> --enable-debug currently configures hotspot to build with the build
>>> target fastdebug. fastdebug isn't really fast since asserts are turned on,
>>> and it can't be used for debugging since the code is optimized, so the name
>>> is a bit misleading. Personally I always use jvmg when debugging and
>>> testing.
>>
>>
>> But the debug build has asserts on too, doesn't it? It should.
>>
>>>
>>> I asked around the Stockholm office and it seems as most people here
>>> would prefer jvmg. It seems as jvmg is very slow on Mac so those who work on
>>> Mac only uses jvmg when debugging for real, not for regular testing, but
>>> otherwise fastdebug is rarely used here.
>>
>>
>> A long time ago, in a land far far away, fastdebug was pretty good on
>> Solaris, using dbx, worked really well, not perfect but pretty good.
>> Since then, Linux was added, and gdb is needed, and Mac too, so we have
>> many native code debuggers to deal with now.
>> Also since then, machines have gotten faster, and disks are fatter and
>> faster, and maybe fastdebug has lost it's benefit.
>> Testing the debug build is pretty impossible, or was, it just was way too
>> slow, maybe 100x slower than a product build.
>> But fastdebug was possible, maybe 10x slower than a product build.
>> So I have always assumed that fastdebug was faster than a debug build.
>> But who knows what the performance is now. I think I was always looking at
>> an entire JDK built with product vs. debug vs. fastdebug
>> not just hotspot, although hotspot was usually the piece that cause
>> performance issues.
>
>
> I admit we probably haven't run a comparison against fastdebug and jvmg
> recently (especially as jvmg was dropped for a while and only came back when
> it was thought we might extend hotspot-express to JDK 5!) but I think the
> hotspot team will be shocked to discover that fastdebug is slower than jvmg.
> fastdebug is supposed to be faster than debug because it still uses the
> OPT_CFLAGS, not DEBUG_CFLAGS, while also allowing a certain level of
> debugging (and having asserts enabled as does jvmg).
>
> David
> -----
>
>> If all this has changed, then we should re-consider what we build and
>> test.
>>
>> I think it was always agreed that the asserts were invaluable, so we need
>> to keep that on in some build.
>>
>> -kto
>>
>>>
>>> Cheers,
>>> /Jesper
>>>
>>
>