fastdebug vs jvmg

[David Holmes]

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
>>
>