JVM parameter setting and analysis

1. summary

In the previous article, I introduced jvm Memory management and garbage collection , Combine these theoretical knowledge , The performance of the system can be improved by reasonably setting parameters .

2. JVM main parameter

2.1. Basic setting parameters

JVM The main meaning of setting parameters

Parameter name	meaning	The default value is	explain
-Xms	Minimum heap size	Physical memory 1/64(<1GB)	Free heap memory is less than 40%（ adopt MinHeapFreeRatio Parameter to adjust the threshold ） when ,jvm Will increase the heap until -Xmx
-Xmx	Maximum heap size	Physical memory 1/4(<1GB)	Free heap memory is greater than 70%（ adopt MaxHeapFreeRatio Parameter to adjust the threshold ） when ,jvm Will reduce the heap until -Xms
-Xmn	The size of the new generation	nothing	finger Eden With two Survivor Sum of space ,Sun The official recommendation is to configure the entire heap as 3/8
-XX:PermSize	Method area （ Forever ） size	Physical memory 1/64	-
-Xss	Stack size per thread	1M	-
-XX:ThreadStackSize	Thread stack size	1M	The main thread uses -Xss Set as primary , Other threads are dominated by this setting , Generally, the default value can be used
-XX:NewRadio	The ratio of the size of the new generation to that of the old generation	nothing	Xms=Xmx And set up Xmn Under the circumstances , This parameter does not need to be set
-XX:SurvivorRatio	Eden Area Survivor The ratio of area size	nothing	-
-XX:LargePageSizeInBytes	Memory page size	128M	Cannot be set too large
-XX:+UseFastAccessorMethods	Whether to use the quick optimization of the original type	nothing	-
-XX:+DisableExplicitGC	Whether to shut down System.gc()	nothing	-
-XX:MaxTenuringThreshold	The maximum age of garbage	nothing	N The new generation resources that have not been recycled will be automatically put into the old generation , Only use serial GC Only when effective
-XX:MaxGCPauseMillis	Maximum pause time	-	The maximum time for each young generation garbage collection
-Xnoclassgc	Jinyong garbage recycling	nothing	-
-XX:SoftRefLRUPolicyMSPerMB	no M Soft reference lifetime in heap memory	1s	-
-XX:PretenureSizeThreshold	Objects over how large are automatically allocated in older generations	0	use Parallel Scavenge GC When is invalid
-XX:TLABWasteTargetPercent	TLAB Occupy eden % of area	1%	-
-XX:+CollectGen0First	FullGC Whether it's first or not YGC	false	-

2.2. Collect related parameters in parallel

JVM parallel GC Setting parameters of

Parameter name	meaning	The default value is	explain
-XX:+UseParGC	Use ParNew The collector is used for new generation collection	false	And -XX:+UseConcMarkSweepGC identical
-XX:ParallelGCThreads	Number of parallel collection threads	-	Specify parallelism GC Number of garbage collection threads under , It is better to configure it as the same as CPU The numbers are equal
-XX:+UseParallelOldGC	Set the older generation collector to Parallel Old	JDK1.6 Start to offer	-
-XX:+UseAdaptiveSizePolicy	Automatically select younger generation size and Survivor The proportion	false	It is recommended to keep the parallel collector open
-XX:GCTimeRatio	Set the garbage collection time as a percentage of the program running time	nothing	Greater than 0 Less than 100, Use Parallel Scavenge Set when
-XX:+UseAdaptiveSizePolicy	Whether to automatically determine the maximum throughput limit according to the current system conditions	false	Use Parallel Scavenge Set when
-XX:+ScavengeBeforeFullGC	Full GC Pre invocation YGC	true	-

2.3. CMS Related parameters

JVM CMS GC Setting parameters of

Parameter name	meaning	The default value is	explain
-XX:+UseConcMarkSweepGC	Enable CMS GC	false	-
-XX:CMSFullGCsBeforeCompaction	How many times GC After that, perform a memory compression （ Defragmentation ）	-	-
-XX:+CMSParallelRemarkEnabled	Lower the mark pause	false	-
-XX+UseCMSCompactAtFullCollection	Whether to enable memory compression （ Defragmentation ）	-	Recommended Opening
-XX:+UseCMSInitiatingOccupancyOnly	Manual trigger CMS	false	prohibit HotSpot Automatic triggering CMS GC
-XX:CMSInitiatingOccupancyFraction	GC Trigger threshold （ percentage ）	1.5 by 68,1.6 by 92	When the memory usage reaches the threshold, it starts GC, Setting up a meeting may lead to failure full GC
-XX:+CMSIncrementalMode	Set to incremental mode	false	Apply to single CPU Next , stay CMS Pause during operation , Continue running the user thread , Next increment GC
-XX:CMSClassUnloadingEnabled	When the garbage collection cleans up the method area, remove those that are no longer used classes	-	-
-XX:+CMSPermGenSweepingEnabled	Whether to clean the method area （ Forever ）	-	The default is not to clean

3. Set rules

3.1. Young generation size choice

1. Response time first applications — Increase the size of the younger generation as much as possible , This also means that the size of the old age will be relatively reduced , So at the same time, we should reduce the number of objects reaching the old age 2. Throughput priority application — As many young people as possible , Parallel garbage collection can be selected , fit 8CPU The above applications

in any case , Remember not to set the new generation too small , Otherwise, it will cause a new generation GC frequent , Even let the new generation objects directly enter the old generation , triggering full GC.

3.2. Old age size choice

1. Response time first applications — Usually used in the old days CMS Concurrent collection , So we can't design too small in the old age , Otherwise, too much memory will cause frequent full GC; If the design is too large , You need a longer collection , Therefore, it is necessary to collect information in combination with concurrency 、 Number of persistent concurrent collections 、 Tradition GC Information 、 The time ratio of the younger generation and the older generation takes into account the specific memory size 2. Throughput priority application — Such an application usually requires a younger generation + Young and old generation , In this way, most short-term objects can be recycled as much as possible , Reduce the number of medium-term objects , In the old age, long-lived objects are stored

3.3. Memory fragmentation in the old days

If you use CMS As a collector for the elderly , So because of the mark removal algorithm he adopted , It usually causes debris , If the final space is insufficient , It will trigger once full GC. In this case , The following configuration is required ： 1. -XX:+UseCMSCompactAtFullCollection — When using concurrent collectors , Turn on the compression of the old 2. -XX:CMSFullGCsBeforeCompaction=0 — When the above configuration is on , How many times is it set here Full GC after , Compress the old generation

3.4. Other matters

1. linux 64 Bit operating system ,jdk Consume more memory , Slower execution , But throughput is bigger 2. XMX and XMS The setting is the same size ,MaxPermSize and MinPermSize The setting is the same size , This can reduce the pressure of the size of the telescoping heap 3. Use CMS The advantage is to use as few new generations as possible , The empirical value is 128M－256M, And then the old generation uses it CMS Parallel collection , This can ensure the low latency throughput of the system . actually cms The collection pause time of is very short ,2G Of memory , about 20－80ms Application pause time 4. When the system stops, it may be GC The problem may also be the problem of the program , multi-purpose jmap and jstack see , perhaps killall -3 java, Then check it out java Console log , Can see a lot of problems . 5. If cache is used , So the old generation should be older 6. When using concurrent recycle , Younger generation is a little smaller , Old age is big , Because old age is a big use of concurrent recycling , Even if the time is long, it will not affect other programs to continue to run , The site doesn't stop

4. Problem analysis — promotion failed

produce promotion failed It may be caused by two reasons ： 1. A new generation of survivor Insufficient , Many objects in the young generation are waiting to be put into survior Memory , the Survivor The space is insufficient , These memories are put into the old generation 2. There is not enough memory in the old age to accept objects from the younger generation

Solve this problem , We should start from these two reasons ： 1. increase Survivor Area size , Set up -XX:SurvivorRatio=1 2. Reduce CMSInitiatingOccupancyFraction（ If set to 70）, Let the older generation be recycled at a lower threshold

It can be inferred that ：eden+from survivor < old gen When memory is left , There will be no promontion faild The situation of , namely ： (Xmx-Xmn)(1-CMSInitiatingOccupancyFraction/100)>=(Xmn-Xmn/(SurvivorRatior+2)) obtain ：CMSInitiatingOccupancyFraction <=((Xmx-Xmn)-(Xmn-Xmn/(SurvivorRatior+2)))/(Xmx-Xmn)100.