[zz]在实践中使用Jstat监控gc情况
zz from http://rdc.taobao.com/blog/qa/?p=1198
性能测试过程中，我们该如何监控java虚拟机内存的使用情况，用以判断JVM是否存在内存问题呢？如何判断JVM垃圾回收是否正常？一般的top指令基本上满足不了这样的需求，因为它主要监控的是总体的系统资源，很难定位到java应用程序。
在项目实践过程中，我们探索和使用了一款新工具－－Jstat。
先秀一下。Jstat是JDK自带的一个轻量级小工具。全称“Java Virtual Machine statistics monitoring tool”，它位于java的bin目录下，主要利用JVM内建的指令对Java应用程序的资源和性能进行实时的命令行的监控，包括了对Heap size和垃圾回收状况的监控。可见，Jstat是轻量级的、专门针对JVM的工具，非常适用。
那，该怎么用呢？
语法结构如下：jstat [Options] vmid [interval] [count]
Options — 选项，我们一般使用 -gcutil 查看gc情况
vmid — VM的进程号，即当前运行的java进程号
interval– 间隔时间，单位为秒或者毫秒
count — 打印次数，如果缺省则打印无数次
下面给出一个实际的例子：
注：由于JVM内存设置较大，图中百分比变化不太明显
图中参数含义如下：
S0 — Heap上的 Survivor space 0 区已使用空间的百分比
S1 — Heap上的 Survivor space 1 区已使用空间的百分比
E — Heap上的 Eden space 区已使用空间的百分比
O — Heap上的 Old space 区已使用空间的百分比
P — Perm space 区已使用空间的百分比
YGC — 从应用程序启动到采样时发生 Young GC 的次数
YGCT– 从应用程序启动到采样时 Young GC 所用的时间(单位秒)
FGC — 从应用程序启动到采样时发生 Full GC 的次数
FGCT– 从应用程序启动到采样时 Full GC 所用的时间(单位秒)
GCT — 从应用程序启动到采样时用于垃圾回收的总时间(单位秒)
S1 — Heap上的 Survivor space 1 区已使用空间的百分比
E — Heap上的 Eden space 区已使用空间的百分比
O — Heap上的 Old space 区已使用空间的百分比
P — Perm space 区已使用空间的百分比
YGC — 从应用程序启动到采样时发生 Young GC 的次数
YGCT– 从应用程序启动到采样时 Young GC 所用的时间(单位秒)
FGC — 从应用程序启动到采样时发生 Full GC 的次数
FGCT– 从应用程序启动到采样时 Full GC 所用的时间(单位秒)
GCT — 从应用程序启动到采样时用于垃圾回收的总时间(单位秒)
上图的示例，红框中，我们可以看到，5次young gc之后，垃圾内存被从Eden space区(E)放入了Old space区(O)，并引起了百分比的变化，导致Survivor space使用的百分比从19.69%(S0)降到10.34%(S1)。有效释放了内存空间。绿框中，我们可以看到，一次full gc之后，Old space区(O)的内存被回收，从36.81%降到35.01%。
图中同时打印了young gc和full gc的总次数、总耗时。而，每次young gc消耗的时间，可以用相间隔的两行YGCT相减得到。每次full gc消耗的时间，可以用相隔的两行FGCT相减得到。例如红框中表示的第一行、第二行之间发生了1次young gc，消耗的时间为52.281-52.252＝0.029秒。
常驻内存区(P)的使用率，始终停留在37.6%左右，说明常驻内存没有突变，比较正常。
如果young gc和full gc能够正常发生，而且都能有效回收内存，常驻内存区变化不明显，则说明java内存释放情况正常，垃圾回收及时，java内存泄露的几率就会大大降低。但也不能说明一定没有内存泄露。
以上，介绍了Jstat按百分比查看gc情况的功能。其实，它还有其它功能，例如加载类信息统计功能、内存池信息统计功能等，那些是以绝对值的形式打印出来的，比较少用，在此就不做介绍。
说了这么多专业知识，大家一定累坏了吧？轻松一刻，卖下广告~~
为了更全面的监控JVM内存使用情况，我们需要引入更强大的工具来进一步分析–JConsole。敬请关注。
Thu, 15 Sep 2022 04:31:05 +0800
This can be the words My group is selecting in all places. Bless you for one's web log, Freezing enroll your site. It's a attractive web log. inch. Remove tree Sacramento
================
This particular is a superb publish We observed because of reveal this. It really is exactly what I needed to determine wish within long term you'll carry on with regard to discussing this type of superb publish. Stump removal sacramento county
================
Bless you designed for presenting modern up-dates concerning the anxiety, As i watch for browse alot more. Kingdom tree service
===============
It a fabulous blog post As i looked at caused by have the application. Advise everything that Need be to find expectation on potential future you certainly will persist designed for writing a great wonderful blog post. what to wear to a pageant
================
Continue the great function, We study couple of articles about this web site as well as I believe that the internet weblog is actually actual fascinating and it has obtained groups associated with wonderful info. how much is it to rent a water slide
================
We suggest which will it's a an amazing blog post to a very good guy, so i'm very happy to find out it. how much does it cost to rent an inflatable obstacle course
================
I think this really is an educational post and it's very helpful and knowledgeable. therefore, I would like to thank you for the efforts you've made in writing this article. Fire damage Spokane wa
================
I do believe that is an educational post and it's very helpful and knowledgeable. therefore, I want to many thanks for the efforts you've made in writing this article. Bounce house rentals Sacramento
================
I believe that is an educational post and it's very useful and knowledgeable. therefore, I wish to thank you for the efforts you've manufactured in writing this article. Bounce house sacramento
================
I do believe this is an educational post and it's very helpful and knowledgeable. therefore, I wish to thanks for the efforts you have manufactured in writing this article. Is SEO worth it in 2022
================
I think this really is an educational post and it's very helpful and knowledgeable. therefore, I would like to thanks for the efforts you've manufactured in writing this article. flower delivery seattle, WA
================
I do believe that is an insightful post and it's invaluable and knowledgeable. therefore, I wish to thanks for the efforts you have produced in writing this article. commercial window tinting sacramento
=================
Whenever I have some free time, I visit blogs to get some useful info. Today, I found your blog with the help of Google. Believe me; I found it one of the most informative blog. sell my house fast seattle washington
Thu, 15 Sep 2022 22:38:26 +0800
You could be allowed to put up manufacturers, except for shortcuts, in the event that they've been recognized not to mention concerning content. [url=https://www.cbs-consulting.com/apac/sap-fiori/]Fiori SAP[/url]
Thu, 29 Sep 2022 14:38:53 +0800
Download NCERT STD-10 Biology Sample Paper 2023 Pdf to know the new exam scheme or question pattern for all formats of exams such as SA1, SA2, FA1, FA2, FA3, FA4 and Assignments held in Term-1 & Term-2 of the Course. NCERT Biology Question Paper Class 10 By following these NCERT 10th Class Biology Model Papers 2023 every students get clarity on the question paper model, asking way, from where most questions repeated, which topics weightage more marks and so on.Download NCERT STD-10 Biology Sample Paper 2023 Pdf to know the new exam scheme or question pattern for all formats of exams such as SA1, SA2, FA1, FA2, FA3, FA4 and Assignments held in Term-1 & Term-2 of the Course.
Fri, 28 Oct 2022 23:14:29 +0800
Wow! This can be a particular of the very useful blogs We have now ever arrive across for this subject. Actually Magnificent. I'm sure also an expert during this topic so I often understand your effort. AdSense Approval
Mon, 16 Jan 2023 01:44:38 +0800
I’m curious to find out what blog platform you are using? I’m experiencing some minor security issues with my latest site and I’d like to find something more secure. Do you have any solutions? 룸알바
Fri, 20 Jan 2023 01:26:37 +0800
Oh my goodness! a great write-up dude. Thanks a ton Nevertheless We are experiencing problem with ur rss . Do not know why Not able to join it. Could there be everyone obtaining identical rss difficulty? Anyone who knows kindly respond. Thnkx Gold99
====================
Great website you got here! Hope you keep updating and make people like me visit! It’ll be in my bookmarks so better update! 로드샵후기
Tue, 24 Jan 2023 22:16:09 +0800
The next occasion I read a weblog, Lets hope it doesnt disappoint me as much as that one. I am talking about, I know it was my method to read, but I really thought youd have some thing fascinating to mention. All I hear is actually a number of whining about something you could fix if you ever werent too busy searching for attention. 성기확대수술
Tue, 31 Jan 2023 01:32:19 +0800
pleasant post, keep up with this interesting work. It really is good to know that this topic is being covered also on this web site so thank for taking time to discuss this! Dallas power bi servcies
Tue, 14 Feb 2023 18:00:53 +0800
Aw, it was an extremely good post. In concept I would like to invest writing this way moreover – spending time and actual effort to make a top notch article… but exactly what can I say… I procrastinate alot by no indicates find a way to get something accomplished. Bilik Sewa
=======================
Receive additional organic exploring fluff to the frizzy hair by employing mousses along with conditioners. 市場推廣
Mon, 31 Jul 2023 17:01:57 +0800
Your blog has piqued a lot of real interest. I can see why since you have done such a good job of making it interesting. I appreciate your efforts very much. <a href="https://vipbox.today/">https://vipbox.today/</a>
Thu, 05 Oct 2023 21:21:04 +0800 I think this is among the most important information for me. And i’m glad reading your article. But want to remark on few general things, The web site style is wonderful, the articles is really nice : D. Good job, cheers 花藝師
Tue, 31 Oct 2023 22:11:18 +0800
Aw, it was an incredibly good post. In idea I would like to put in writing similar to this additionally – spending time and actual effort to have a really good article… but what things can I say… I procrastinate alot and also by no means manage to go completed. 訂花
==================
Impressive piece of content! The way we wish had that understanding. I’m hoping to study a lot more on your side. You will find you might amazing details combined with idea. I’m sure tremendously contented with that info. 訂花推介
==================
I truly appreciate this post. I have been looking everywhere for this! Thank goodness I found it on Bing. You’ve made my day! Thx again! 開張花籃花牌
==================
I’m often to running a blog and i actually respect your content. The article has actually peaks my interest. I am going to bookmark your site and keep checking for new information. 玫瑰花
Sat, 11 Nov 2023 21:49:15 +0800
I do agree with all the ideas you’ve presented in your post. They are really convincing and will certainly work. Still, the posts are too short for newbies. Could you please extend them a bit from next time? Thanks for the post. احسن جامعة مصر
Thu, 16 Nov 2023 23:48:28 +0800
i like Californication because of the pretty girls and the nice music that it imparts on the show,, https://lightinfitness.com/
Fri, 24 Nov 2023 01:03:14 +0800
So, naturally, with these two together, movie magic is accomplished. 手機回收
========================
we have different sectional sofas at home, i find them very comfortable and easy to setup’ iphone trade in邊間好
========================
Helpful information. Fortunate me I found your web site unintentionally, and I am stunned why this twist of fate did not took place in advance! I bookmarked it. iphone13回收價
=======================
You made certain nice points there. I did a search on the matter and found most people will go along with with your blog. iphone14 pro max回收價
Wed, 17 Jan 2024 21:39:54 +0800
I always leave 오피타임 feeling rejuvenated and stress-free. Highly recommended!
Thu, 18 Jan 2024 04:55:44 +0800
제주안마's blend of nature and therapeutic techniques on Jeju Island is a gateway to serenity, offering respite from the hectic pace of life.
Mon, 29 Jan 2024 05:13:20 +0800
I think other web-site proprietors should take this web site as an model – very clean and wonderful style and design, as well as the content. You’re an expert in this topic! 고카지노보증