Bug in G1GC it performs Full GC when code cache is full resulting in overkill

[Martin Makundi]

Hi!

> On the subject of cmd line options ...

Thanks for the detailed feedback, here is what we based our decisions upon:

> Here's a list of options that I think look a bit questionable, and I'd like to understand why you feel the need to set them:

They are not very clearly documented, so there are a lot of 'shotgun' options.

> -XX:+UseFastAccessorMethod  (the default is disabled)

Fast sounds good, the description of it is "Use optimized versions of
Get<Primitive>Field" which sounds good. I see no harm in this.

> -XX:+UseNUMA  (Are you running a JVM that spans NUMA memory nodes?  Or, do you have multiple JVMs running on a NUMA or non-NUMA system?)

Single JVM 64bit Linux, I do not know the technical details, but
switched on based on this sentence:
NUMA Performance Metrics

When evaluated against the SPEC JBB 2005 benchmark on an 8-chip
Opteron machine, NUMA-aware systems showed the following performance
increases:
    32 bit – About 30 percent increase in performance with NUMA-aware allocator
    64 bit – About 40 percent increase in performance with NUMA-aware allocator

> -XX:+UseStringCache (Do you have evidence that this helps?  And, do you know what it does?)

I assume it is some sort of string interning solution. Don't know
exactly what it does, but our application uses high amount of
redundant strings, smaller memory footprint is a good idea. Again,
very little documentation about this available but seems
straightforward. Haven't benchmarked it personally.

> -XX:CMSInitiatingOccupancyFraction=70 (This is applicable to CMS GC, and not applicable to G1 GC)

Again, not documented thoroughly where it applies and where not, jvm
gave no warning/error about it so we assumed it's valid.

> -XX:GCPauseIntervalMillis=10000 (Would like to understand the justification for setting this, and to a 10 second value. This will impact G1.)

I understand what matters is the ratio
MaxGCPauseMillis/GCPauseIntervalMillis and a larger
GCPauseIntervalMillis makes it less aggressive and thus less overhead?

> -XX:InitiatingHeapOccupancyPercent=0  (You realize this will force G1's concurrent cycle to run continuously?)

Yes, that's what we figured out, we don't want it to sit lazy and end
up in a situation where it is required to do a Full GC. This switch
was specifically chosen in a situation we had a memory leak and tried
to aggressively fight against it before we found the root cause. Maybe
we should try without this switch now, and see what effect it has.

> -Xmaxf1 (I've never seen this used before, can you share what you it does and what you expect it to do?)

Again, referring to previous memory leak issues, we did not want the
application to fight with other applications for available memory.
Xmaxf1 keeps memory reservation fixed to initial value which is equal
to maximum value.

> -noclassgc (This is rarely needed and haven't seen an app that required it for quite some time)

Jvm 1.6 stopped the world for couple of minutes several times per day
while unloading classes, so we used noclassgc to disable that. We do
not know if this is necessary for latest 1.7 to avoid class unload
pause, but we continued to use this switch and found no harm in it.
Can't afford testing that in production ;)

> These you don't need to set as they are the default with 1.7.0_21 when you specify -XX:+UseG1GC, hence you can remove them:

Some of them are set explicitly just to keep track amidst jvm upgrades.

> -XX:+UseAdaptiveSizePolicy
> -XX:+UseCompressedOops (this gets auto-enabled based on the size of the Java with 64-bit JVMs --- and you might realize slightly better performance if you can run with -Xmx26g / -Xms26g, that should give you zero base compressed oops)

Thanks, good to know, will try that. Is it exactly 26g or bits more or
bits less?

> -XX:+ UseGCOverheadLimit
> -XX:+ReservedCodeCacheSize=48, that is the default for 7u21.  You might consider setting it higher if you have the available space, and more importantly if you think you're running out of code space.

For our sun jvm linux 64bit 48m is maximum, jvm won't start if higher value.


**
Martin

>
>
> On May 26, 2013, at 10:20 AM, Martin Makundi wrote:
>
>> Sorry, forgot to mention, using:
>>
>> java version "1.7.0_21"
>> Java(TM) SE Runtime Environment (build 1.7.0_21-b11)
>> Java HotSpot(TM) 64-Bit Server VM (build 23.21-b01, mixed mode)
>>
>> Linux version 3.0.1.stk64 (dfn at localhost.localdomain) (gcc version
>> 4.5.1 20100924 (Red Hat 4.5.1-4) (GCC) ) #1 SMP Sat Aug 13 12:53:46
>> EDT 2011
>>
>> -Dclassworlds.conf=/usr/share/maven/maven/bin/m2.conf
>> -Dmaven.home=/usr/share/maven/maven
>> -Duser.timezone=EET
>> -XX:+AggressiveOpts
>> -XX:+DisableExplicitGC
>> -XX:+ParallelRefProcEnabled
>> -XX:+PrintGCDateStamps
>> -XX:+PrintGCDetails
>> -XX:+PrintHeapAtGC
>> -XX:+UseAdaptiveSizePolicy
>> -XX:+UseCompressedOops
>> -XX:+UseFastAccessorMethods
>> -XX:+UseG1GC
>> -XX:+UseGCOverheadLimit
>> -XX:+UseNUMA
>> -XX:+UseStringCache
>> -XX:CMSInitiatingOccupancyFraction=70
>> -XX:GCPauseIntervalMillis=10000
>> -XX:InitiatingHeapOccupancyPercent=0
>> -XX:MaxGCPauseMillis=500
>> -XX:MaxPermSize=512m
>> -XX:PermSize=512m
>> -XX:ReservedCodeCacheSize=48m
>> -Xloggc:gc.log
>> -Xmaxf1
>> -Xms30G
>> -Xmx30G
>> -Xnoclassgc
>> -Xss4096k
>>
>>
>> **
>> Martin
>>
>> 2013/5/26 Charlie Hunt <chunt at salesforce.com>:
>>> Which version of the JDK/JRE are you using?
>>>
>>> One of the links you referenced below was using JDK 6, where there is no official support for G1. The other link suggests it could have been RMI DGC or a System.gc().
>>>
>>>
>>>
>>> Sent from my iPhone
>>>
>>> On May 25, 2013, at 11:43 PM, "Martin Makundi" <martin.makundi at koodaripalvelut.com> wrote:
>>>
>>>> it occurs daily.
>