Monday, December 24, 2018

Command line tools every (Java) developer should know

In this post I will go through some useful command line tools to debug and tune your service. I will show a sample output and description of the most important parts of the output, feel free to read the manual pages for more details about the tools and supported flags to tune their output.


This system tool is very useful to view and monitor statistics of your system processes, memory, paging, block IO, disks, and CPU. Its useful to check IO/CPU utilization and spot any bottlenecks.


$ vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0      0 12583088 119480 1590740    0    0   406    72  470 1035 10  3 87  0  0
 0  0      0 12582832 119480 1590740    0    0     0     0  451 1562  1  0 98  0  0
 0  0      0 12579484 119480 1590732    0    0     0     0  408 1345  1  0 98  0  0

The above command is to show stats with 1 second delay.

Processes: Amount of runnable and un-interruptible processes
Memory: Free/cache memory and memory used as buffers and the amount of virtual memory used (swpd)
Swap: Swap in and out (from disk)
IO: Blocks in (READ) and out (WRITE) from disk
System: Number of interrupts per second and context switches
CPU: Time running user code vs system (kernel) code, idle time, and waiting for IO



This system tool is very useful to view and monitor IO statistics of your system.


$ iostat -xm 5
Linux 4.15.0-42-generic

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           5.27    0.02    1.59    0.15    0.00   92.97

Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               0.00     5.29   20.82    6.42     0.43     0.11    40.69     0.02    1.18    0.25    4.20   0.14   0.39

The -x flag is to show extended statistics and -m is to view stats in megabytes per second. Of course some of the information above can also be viewed by the previous tool `vmstat`.

%user, %system, %iowait, %idle: The percentage of CPU spent running user code, system code, waiting for IO, and idle
rrqm/s, wrqm/s: The number of (read/write) requests merged per second that were queued to the device
r/s, w/s, rMB/s, wMB/s: Reads/Writes (after merge) per seconds, MBs per seconds (read/write)
await, r_await, w_await: Average time in ms for IO requests to the device to be served, average time in ms for read, average time in ms for write


This tool isn't installed by default in linux systems but can be installed using package managers or downloaded from sourceforge. It basically does the same as previous tools but for network monitoring.


$ nicstat 5
    Time      Int   rKB/s   wKB/s   rPk/s   wPk/s    rAvs    wAvs %Util    Sat
20:10:52       lo    0.33    0.33    3.98    3.98   85.25   85.25  0.00   0.00
20:10:52   wlp2s0    6.18    1.85   12.23    9.03   517.6   210.0  0.00   0.00

Prints network statistics ever 5 seconds.

Int: Interface name
rKB/s, wKB/s: KBs Read(Received) Write(Sent)
rPk/s, wPk/s: Packets received/sent
rAvs, wAvs: Average packet size received/sent
%Util, Sat: Percentage utilization of the interface, Saturation (errors/second)


This tool and the coming few ones come along with the JDK and can be used to send diagnostic commands to a running JVM.


$ jcmd 8880 VM.uptime # 8880 is the process ID, VM.uptime is to check uptime of the running java process

163.883 s

$ jcmd 8880 VM.system_properties # View system properties of a single java process

#Mon Dec 24 20:32:01 CET 2018 Runtime Environment
java.vm.vendor=Oracle Corporation
path.separator=\: 64-Bit Server VM
..... # and more .....

$ jcmd 8880 VM.version

OpenJDK 64-Bit Server VM version 25.191-b12
JDK 8.0_191

$ jcmd 8880 VM.command_line # The command line of the process was started

VM Arguments:
java_command: HelloWorld -Dse7so=29 -client
java_class_path (initial): .
Launcher Type: SUN_STANDARD

$ jcmd 8880 VM.flags # Enable flags - try with [-all]

-XX:CICompilerCount=3 -XX:InitialHeapSize=262144000 -XX:MaxHeapSize=4175429632 -XX:MaxNewSize=1391460352 -XX:MinHeapDeltaBytes=524288 -XX:NewSize=87031808 -XX:OldSize=175112192 -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC

$ jcmd 8880 Thread.print # Thread dump - see also jstack tool below
# Very long output here that can be saved to a file for later analysis

$ jcmd 8880 #
Command executed successfully

$ jcmd 8880 GC.class_histogram # Show heap histogram ([C is character array, [B is byte array, [I is integer array)

 num     #instances         #bytes  class name
   1:          1157        1614520  [C
   2:           131         216072  [I
   3:           474          53992  java.lang.Class
   4:          1145          27480  java.lang.String
   5:           531          26632  [Ljava.lang.Object;
   6:            20          25264  [B
   7:           378           9072  java.util.LinkedList$Node
   8:           211           6752  java.util.HashMap$Node
   9:           190           6080  java.util.LinkedList
  10:            75           5400  java.lang.reflect.Field
  11:           256           4096  java.lang.Integer
  12:            93           3720  java.lang.ref.SoftReference
  13:           111           3552  java.util.Hashtable$Entry
  14:             7           2632  java.lang.Thread
  15:            12           1984  [Ljava.util.HashMap$Node;
  16:            51           1896  [Ljava.lang.String;
  17:            38           1824  sun.util.locale.LocaleObjectCache$CacheEntry
  18:            47           1504  java.util.concurrent.ConcurrentHashMap$Node
  19:            23           1472
  20:            14           1120  [Ljava.util.WeakHashMap$Entry;
  21:            14           1120  java.lang.reflect.Constructor
  22:             2           1064  [Ljava.lang.invoke.MethodHandle;
  23:             1           1040  [Ljava.lang.Integer;
  24:            26           1040
  25:             6            992  [Ljava.util.Hashtable$Entry;
  26:            15            840  java.lang.Class$ReflectionData
  27:            21            840  java.lang.ref.Finalizer


In fact this tool is a GUI tool but it worth mentioning as it provides very useful information to monitor and control java applications.

It views information about Memory, CPU, Threads, and Classes loaded.


Provides heap dumps and other information about JVM memory usage.


$ jmap -dump:live,format=b,file=/tmp/dump.hprof 9293
Dumping heap to /tmp/dump.hprof ...
Heap dump file created

Generates a dump from a live process, in binary format and saves it to /tmp/dump.hprof - you can then analyze it with any heap dump analyzer.


Useful to generate thread dumps with full stack traces.


$ stack -l > /tmp/myapp.tdump

You can then analyze the thread dump using any thread dump analyzer.


This is also a GUI tool but very useful to monitor your java application. Its man page describes it as all-in-one java troubleshooting tool and in fact it is very nice and can be very useful along with the above commands. you can also use to analyze heap and thread dumps generated earlier using jmap and jstack.


Is one of the best tools to view very useful stats about GC, JIT compiler, and so many other things to monitor.


$ jstat -gcutil -t 9293 100ms
Timestamp         S0     S1     E      O      M     CCS    YGC     YGCT    FGC    FGCT     GCT   
         5452.8   0.00   0.00  77.88   0.86  96.16  84.57     10    0.056    10    0.295    0.351
         5453.0   0.00   0.00  77.88   0.86  96.16  84.57     10    0.056    10    0.295    0.351

The above shows GC information (the -t to show timestamps) every 100ms.

S0, S1: Survivor spaces 0 and 1 utilization as a percentage of the space's current capacity
E, O, M: Eden space, Old space, Metaspace utilization as percentage of the space's current capacity
CCS: Compressed class space utilization as a percentage
YGC, YGCT: Number of young generation GC events and young generation gc time
FGC, FGCT: Number of full GC events, and full GC time
GCT: Total garbage collection time

When the Eden space percentage goes up this triggers a young generation minor GC event which then will reduce its size and the S0, S1 will be swapped and of course Old generation increases a bit (some objects move from Y to O) - Full GC events are triggered when the Old generation percentage goes up  and as a result its size is reduced and fragmented.

$ jstat -class -t 9293 1s
Timestamp       Loaded  Bytes  Unloaded  Bytes     Time   
         5845.2   3602  7665.3       13    15.8       1.33
         5846.2   3602  7665.3       13    15.8       1.33
         5847.2   3602  7665.3       13    15.8       1.33
         5848.2   3602  7665.3       13    15.8       1.33

The same command but to view class loader statistics every 1 second.

Loaded: Number of classes loaded
Bytes: Number of bytes loaded
Unloaded: Number of classes unloaded
Bytes: Number of bytes unloaded
Time: Time spent performing class loading and unloading

$ jstat -printcompilation -t 9293 1s
Timestamp       Compiled  Size  Type Method
         6067.7     3744    684    1 java/util/ComparableTimSort sort
         6068.7     3744    684    1 java/util/ComparableTimSort sort
         6069.7     3744    684    1 java/util/ComparableTimSort sort

The same command but to view Java HotSpot VM compiler method statistics.

Compiled: Number of compilation tasks performed on the method
Size: Size of code byte code (not binary code)
Type: Compilation type of the most recently compiled method 
Method: Class name and method name identifying the most recently compiled method
$ jstat -compiler -t 9293 1s
Timestamp       Compiled Failed Invalid   Time   FailedType FailedMethod
         6378.9     3753      3       0    12.54          1 org/netbeans/lib/profiler/server/ProfilerRuntimeObjLiveness traceObjAlloc
         6380.0     3753      3       0    12.54          1 org/netbeans/lib/profiler/server/ProfilerRuntimeObjLiveness traceObjAlloc
         6381.0     3753      3       0    12.54          1 org/netbeans/lib/profiler/server/ProfilerRuntimeObjLiveness traceObjAlloc
         6382.0     3753      3       0    12.54          1 org/netbeans/lib/profiler/server/ProfilerRuntimeObjLiveness traceObjAlloc

Java HotSpot VM Just-in-Time compiler statistics.

Compiled: Number of compilation tasks performed
Failed: Number of failed compilation tasks
Invalid: Number of invalidated compilation tasks
Time: Time spent on compilation
FailedMethod: The most recent failed method to compile

The last two examples are a bit hard to understand if you don't really know the basics of JIT and how it works .. you can go ahead and read what happens behind the scene to realize the importance of both examples and how you can use them to diagnose possible performance issues in your services.

Those are the most useful tools from my point of view to monitor, diagnose, troubleshoot your services .. please comment with other tools you might see useful but not stated above.


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