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Garbage CollectionDanny Angus

Introduction• Student loans,

amongst other things, run B2B applications implementing government policy in the UK.

• We process 900,000+ complex loan applications in the six months from March to September every year.

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Introduction• We have to tune

performance, our funding stakeholders demand we provide value for money. (As a tax payer so do I).

• You should tune for performance too.

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Myth

• Memory management is one of the things Java is supposed to do for us.

• We do have to be good citizens• And sometimes it still fails to live up

to our expectations.

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Just take a look at these comments from APR, and imagine the code. Glad you don’t have to write it? I know I am.

/* Round up the block size to the next boundary, but always

* allocate at least a certain size (MIN_ALLOC).*//* Find the index for this node size by* dividing its size by the boundary size*//* First see if there are any nodes in the area we know* our node will fit into.*//* Walk the free list to see if there are* any nodes on it of the requested size** NOTE: an optimization would be to check* allocator->free[index] first and if no* node is present, directly use* allocator->free[max_index]. This seems* like overkill though and could cause* memory waste.*//* If we found nothing, seek the sink (at index 0), if* it is not empty.*//* Walk the free list to see if there are* any nodes on it of the requested size*//* If we haven't got a suitable node, malloc a new one* and initialize it.*/

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So what do we have

• A load of theory but no silver bullet (but I think Sun would like the “AgressiveHeap” option to work for most of us)

• No “API” which we (the hardworking geeks in the field) can use to even hint about what our applications are going to be doing.

• A few general heap options• A lot of “secret” knowledge

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How to keep up with the work

• Suns JVM uses “generational” heap management

• Partions for old & young• Java can create a lot of new objects, • For long running processes under

steady heavy load that means a real lot.

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The Generations

• Young generation• New space, when you create an object memory

here is allocated.

• Tenured generation• If it last longer than a wee while it is moved

here

• Permanent generation• Stuff (classes loaded mainly) that won’t be

thrown away

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And there’s more to the young

• Creation space• Two survivor spaces

• Objects are promoted from survivor space when they survive a certain number of collections. You can set this number.

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A Generalisation about types

• Mark and sweep• Start in the VM and follow all of the object

references you can reach, • mark each object• Remove the ones you cant reach

• Stop and copy• Stop execution• Copy all the objects you can reach into a new

space

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Mark and sweep

• Lots of work to do all that marking• Leads to fragmentation• You can sweep concurrent with execution• This is good if you have plenty of CPU

and want to avoid pauses.• Still need to defrag now and then though.• More garbage == more work

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Stop and copy

• You need twice as much memory• Pauses everything for the whole time

it takes• De-fragments the space every time• More survivors == more work

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JRE 1.4

• Copying collector – default for the young space – aka “minor collection”• Parallel copying –XX:+UseParNewGC• Parallel scavenge –XX:+UseParallelGC

• Mark – compact – default for tenured space – major collection• Concurrent collector –XX:

+UseConcMarkSweepGC

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What is your goal?

• “They” expect you to choose betweenThroughput & Response time, or speed v no pauses.

• But if you’re like me you’ll want everything.

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The compromise

• Balance the time spent in collections against the number of collections.

• Get the maximum benefit from each collection.

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Some more clues

• Space fills to its threshold, and is cleaned right out.

• In a steady state process there will be an apparent base load in a newly cleaned space, you can measure this and size the spaces.

• Then you can pick the collector that least intrusively keeps up with the rate at which you use the space.

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So how do you measure?

• -loggc:<filepath>• -XX:PrintGCDetails• -verbose:gc

[GC 0.000: [DefNew: 512K->64K(576K), 0.0051493 secs] 512K->155K(1984K), 0.0053630 secs]

• Draw the graph• You’re looking for a sawtooth shape• The peaks and troughs should reach the same

point every time.

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•Pink is before, blue after•See the small amount cleaned at each small collection•And the big amount cleaned in the major collection•What does this suggest? Young space is too small•Time line – there is no time on this graph•If the whole think took a day do we have too few major collections•If this whole thing is a minute do we have too many•Context is important

Illustration

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If the heap in use grows

• Reduce the frequency of collections by increasing the young space

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•This time the young generation is more cleaned•Fewer promotions•Tenured space in use after the major collection•If this continues the size of the heap will increase•And pauses will be bigger

Another example

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•Logarythmic early growth•Steady young size and promotion•Would we want the lines to be more horizontal?•Only you can tell, balance memory use vs CPU •Is this under load or is your normal peak load worse?•Optimise for normal peak = = efficient•Optimise for occasional peak = = robust

Tie it back to real events

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If major collections are too frequent

• Increase the size of the heap a bit..

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Don’t overdo it

• If the tenured space is too big you’ll have longer pauses for the tenured collection

• Reduce the number of promtions by sizing the young generation.

• -XX:NewSize• -XX:MaxNewSize• -XX:SurvivorRatio

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Size the young gen

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Use Subtraction… [GC 83473.842: [DefNew: 98814K-

>7027K(104832K), 0.0829770 secs] 704440K->612653K(1036928K), 0.0831090 secs]

[GC 83475.633: [DefNew: 100210K->3604K(104832K), 0.0963160 secs] 705836K->613349K(1036928K), 0.0964500 secs]

• 696k promoted.

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Time, don’t forget it

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A real default profile

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Here’s one I prepared earlier

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Myth exploded

• “Make –Xmx and –Xms the same size”

• If you have plenty of RAM make –Xmx big to cope with the unexpected

• But keep –Xms small to manage collection frequency and size

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Tip

• Never let the OS swap out the Java heap

• This is because marking involves traversing the whole heap in unpredictable patterns

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Size the Young generation

• Make those collections work for you• New size, new ratio balance the eden

space and the survivor spaces • You want as many of your short lived

objects to live and die in one collection, without letting the collection get too onerous of course

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Size the permanent space

• Size it just a bit bigger than you need

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Myth exploded

• “allocations which can’t be made in permanent space will be made from tenured space”

• In practice a compacting collection occurs, and if it doesn’t release enough concurrent memory in the perm space the jvm spins up to 100% re-trying the compaction.

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Parallel and concurrent• Might make sense if you have plenty of

cpu• Uses more cpu, but no pauses• Except that you still get de-fragmentation

pauses, threads have to be parked before objects can be moved and references re-written.

• Marking takes a while, and then you have to collect. This may be too slow to clear down during peaks.

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Aggressive heap

• The silver bullet?

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Some more stuff• -Xincgc / -Xnoincgc• -XX:ParallelGCThreads=<desired number>• -XX:+UseCMSCompactAtFullCollection• -XX:MaxTenuringThreshold=0• -XX:SurvivorRatio=1024• -XX:SoftRefLRUPolicyMSPerMB=10000 • -XX:PretenureSizeThreshold=<byte size> • -XX:+DisableExplicitGC • -XX:CMSInitiatingOccupancyFraction=<percent>• -XX:+UseCMSInitiatingOccupancyOnly• -XX:CMSFullGCsBeforeCompaction=1

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Sizes• Xmsvalue• -Xmxvalue• -Xmnvalue• -XX:MinHeapFreeRatio=minimum• -XX:MaxHeapFreeRatio=maximum• -XX:NewRatio=ratio• -XX:NewSize=size• -XX:MaxNewSize=size• -XX:MaxPermSize=<desired size> • -XX:+AggressiveHeap• -XX:+UseAdaptiveSizePolicy

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Conclusion

• If you spend too much time doing this you probably need to buy hardware.

• But it is worth doing now and again.• Thanks to Student Loans for letting

me talk frankly about our problems.

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Questions?

•http://people.apache.org/~danny

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