10. Find out the JVM operation status

Find out JVM Health of , One of the most useful ways , Is the use of jstat command .

First, through jps You can find what you want java Process corresponding to PID, And then execute jstat –gc PID, You can see this Java process （ In fact, the essence is one JVM） Memory and GC The situation ：

function jstat –gc PID after , You will see the following , Explain to you ：

S0C： This is a From Survivor The size of the area

S1C： This is a To Survivor The size of the area

S0U： This is a From Survivor The size of memory currently used by the area

S1U： This is a To Survivor The size of memory currently used by the area

EC： This is a Eden The size of the area

EU： This is a Eden The size of memory currently used by the area

OC： This is the size of the old age

OU： This is the current memory size used in the old age

MC： This is the method area （ Forever 、 Metadata area ） Size

MU： This is the method area （ Forever 、 Metadata area ） The current memory size used

YGC： This is what the system has done so far Young GC frequency

YGCT： This is what the system has done so far Young GC The total time of

FGC： This is what the system has done so far Full GC frequency

FGCT： This is what the system has done so far Full GC The total time of

GCT： This is all. GC The total time of

And through jstat –gc PID 1000 10, Every 1 Seconds to update the latest line jstat Statistics , A total of execution 10 Time ：

adopt jstat The main information we need to get is ： The growth rate of new generation objects ,Young GC The trigger frequency of ,Young GC Time consuming , Every time Young GC How many objects survived , The growth rate of objects in the old age ,Full GC The trigger frequency of ,Full GC Time consuming .

Then we use this information , Allocate memory space reasonably , Try to keep the object in the younger generation instead of the older generation , Avoid frequent Full GC. This is to JVM Best performance optimization .

1、 The growth rate of new generation objects ：

Such as through jstat –gc PID 1000 10 command , every other 1 Second update jstat Statistics , For example, the first second is displayed Eden Area occupied 250M, The second second second displays Eden Area occupied 255M, The third second displays Eden Area occupied 259M, Then we can roughly estimate Eden The object growth rate of the zone is ： Every time 1 Second growth 5M.

2、Young GC The trigger frequency and time consumption of each time ：

Or the example above , such as Eden The total size of the area is 1G, If you add 5M, So about 200s, in other words 3 branch 20 Every second Young GC. Every time Young GC The time consumption can be based on the operation of the system so far Young GC frequency , And the system so far Young GC Total time , Calculate every time Young GC Time consuming .

3、 Every time Young GC After that, how many objects are surviving and entering the old generation ：

It needs to be monitored many times Young GC,Young GC How many objects are put into Survivor District , How many new objects have the elderly generation added , Get the growth rate of objects in the old age .

4、Full GC Trigger frequency and time consumption of ：

Once you know the growth rate of the old age , Then you only need to know the total size of the old age , You can easily infer Full GC The trigger frequency of . Every time Full GC The time consumption can be based on the operation of the system so far Full GC frequency , And the system so far Full GC Total time , Calculate every time Full GC Time consuming .