Paul Hohensee wrote:
You can try out compressed pointers in 6u14. It just won't be quite as fast as the version that's going into 6u18. 6u14 with compressed pointers will still be quite a bit faster than without.
One of the gc guys may correct me, but UseAdaptiveGCBoundary allows the vm to ergonomically move the boundary between old and young generations, effectively resizing them. I don't know if it's bit-rotted, and I seem to remember that there wasn't much benefit. But maybe we just didn't have a good use case.
Also, it's ParallelGC-only, IIRC.
What I meant by the last paragraph was that with the tenuring threshold set at 15 (which is what the log says), and with only 7 young gcs in the log, we can't see at what age (or if) between 8 and 15 the survivor size goes down to something reasonable. If it doesn't, it might be worth it to us to revisit increasing the age limit for 64-bit.
Paul, the problem in Jeff's case is that even at age 1 he copies 1GB or so. So, maybe, setting a small MTT and having more CMS cycles might be the right option for him. Tony
jeff.lloyd@algorithmics.com wrote:
Thanks for your response Paul.
I'll take another look at the parallel collector.
That's a good point about the -XX:+UseCompressedOops. We started off with heaps bigger than 32G so I had left that option out. I'll put it back in and definitely try out 6u18 when it's available.
What about the option -XX:+UseAdaptiveGCBoundary? I don't see it referenced very often. Would it be helpful in a case like mine?
I'm not sure I understand your last paragraph. What is the period of time that you would be interested in seeing?
Jeff
-----Original Message----- From: Paul.Hohensee@Sun.COM [mailto:Paul.Hohensee@Sun.COM] Sent: Friday, September 11, 2009 1:23 PM To: Tony Printezis Cc: Jeff Lloyd; hotspot-gc-use@openjdk.java.net Subject: Re: Young generation configuration
Another alternative mentioned in Tony and Charlie's J1 slides is the parallel collector. If, as Tony says, you can make the young gen large enough to
avoid promotion, and you really do have a steady state old gen, then which old gen collector you use wouldn't matter much to pause times, given that young gen pause times seem to be your immediate problem.
It may be that you just need more hardware threads to collect such a big
young gen too. You might vary the number of gc threads to see how that affects collection times. If there's significant differences, then you need more hardware threads, i.e., a bigger machine.
You might also try using compressed pointers via -XX:+UseCompressedOops. That should cut down the total survivor size significantly, perhaps enough to that your current hardware threads can collect significantly faster.
Heap size will be limited to < 32gb, but you're app will probably fit. A more efficient version of compressed pointers will be available in 6u18, btw.
I notice that none of your logs shows more than age 7 stats even though the tenuring threshold is 15. It'd be nice to see if anything dies before then.
Paul
Tony Printezis wrote:
Jeff,
Hi. I had a very brief look at your logs. Yes, your app does seem to need to copy quite a lot (I don't think I've ever seen 1-2GB of data being copied in age 1!!!). From what I've seen from the space sizes, you're doing the right thing (i.e., you're consistent with what we talked about during the talk): you have quite large young gen and a reasonably sized old gen. But the sheer amount of surviving objects is
what's getting you. How much larger can you make your young gen? I
think
in this case, the larger, the better. Maybe, you can also try MaxTenuringThreshold=1. This goes against our general advice, but this
might decrease the amount of objects being copied during young GCs, at
the expense of more frequent CMS cycles...
Tony
jeff.lloyd@algorithmics.com wrote:
Hi,
I'm new to this list and I have a few questions about tuning my young
generation gc.
I have chosen to use the CMS garbage collector because my application
is a relatively large reporting server that has a web front end and therefore needs to have minimal pauses.
I am using java 1.6.0_16 64-bit on redhat 5.2 intel 8x3GHz and 64GB
ram.
The machine is dedicated to this JVM.
My steady-state was calculated as follows:
- A typical number of users logged in and viewed several
reports
- Stopped user actions and performed a manual full GC
- Look at the amount of heap used and take that number as
the
steady-state memory requirement
In this case my heap usage was ~10GB. In order to handle variance or
spikes I sized my old generation at 15-20GB.
I sized my young generation at 32-42GB and used survivor ratios of 1,
2, 3 and 6.
My goal is to maximize throughput and minimize pauses. I'm willing
to
sacrifice ram to increase speed.
I have attached several of my many gc logs. The file gc_48G.txt is just using CMS without any other tuning, and the results are much worse than what I have been able to accomplish with other settings. The best results are in the files gc_52G_20Gold_32Gyoung_2sr.txt and gc_57G_15Gold_42Gyoung_1sr.txt.
The problem is that some of the pauses are just too long.
Is there a way to reduce the pause time any more than I have it now?
Am I heading in the right direction? I ask because the default settings are so different than what I have been heading towards.
The best reference I have found on what good gc logs look like come from brief examples presented at JavaOne this year by Tony Printezis and Charlie Hunt. But I don't seem to be able to get logs that resemble their tenuring patterns.
I think I have a lot of medium-lived objects instead of nice short-lived ones.
Are there any good practices for apps with objects like this?
Thanks,
Jeff
This email and any files transmitted with it are confidential and proprietary to Algorithmics Incorporated and its affiliates ("Algorithmics"). If received in error, use is prohibited. Please destroy, and notify sender. Sender does not waive confidentiality or privilege. Internet communications cannot be guaranteed to be timely,
secure, error or virus-free. Algorithmics does not accept liability for any errors or omissions. Any commitment intended to bind Algorithmics must be reduced to writing and signed by an authorized signatory.
------------------------------------------------------------------------
_______________________________________________ hotspot-gc-use mailing list hotspot-gc-use@openjdk.java.net http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
-------------------------------------------------------------------------- This email and any files transmitted with it are confidential and proprietary to Algorithmics Incorporated and its affiliates ("Algorithmics"). If received in error, use is prohibited. Please destroy, and notify sender. Sender does not waive confidentiality or privilege. Internet communications cannot be guaranteed to be timely, secure, error or virus-free. Algorithmics does not accept liability for any errors or omissions. Any commitment intended to bind Algorithmics must be reduced to writing and signed by an authorized signatory. --------------------------------------------------------------------------
_______________________________________________ hotspot-gc-use mailing list hotspot-gc-use@openjdk.java.net http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
-- --------------------------------------------------------------------- | Tony Printezis, Staff Engineer | Sun Microsystems Inc. | | | MS UBUR02-311 | | e-mail: tony.printezis@sun.com | 35 Network Drive | | office: +1 781 442 0998 (x20998) | Burlington, MA 01803-2756, USA | --------------------------------------------------------------------- e-mail client: Thunderbird (Linux) _______________________________________________ hotspot-gc-use mailing list hotspot-gc-use@openjdk.java.net http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use