Archive for the ‘Uncategorized’ Category

Concurrent Modification Exception

Monday, February 8th, 2010

I ran into a ConcurrentModificationException (CME) during stress testing.
What does CME actually mean?
It means that you’ve updated your Collection while you’ve been iterating over it (usually in a multi-threaded fashion, but it can occur in a single thread that updates while iterating).

A few more things to know about CME:
Best effort detection - If you see a CME printout, first off, consider yourself lucky, CMEs are thrown only in best effort. In another universe, the concurrent modification would not have been detected, causing your collection to become corrupted, instead of fast-failing with a CME.

IDing the problem – Like deadlocks, CME’s are easy to pinpoint once you inspected the exception’s stack trace.

Getting around it:

  1. ListIterator
    If you’re single threaded, consider solving the CME by manipulating the collection via the ListIterator interface instead of directly.
    Advantages – simple.
    Drawbacks – suitable for a single thread model.
  2. Synchronizers
    Use locks to obtain mutual exclusion while doing collection R/W operations.
    Advantages – easy to code.
    Drawbacks – lock overhead for reading operations.
  3. Copy-on-write
    Take advantage of the Java.util.concurrent collections like: CopyOnWriteArrayList, CopyOnWriteArraySet. If you require a map then grab CopyOnWriteMap from Apache (this guys have been doing Sun’s dirty work for years now).
    Advantages – very good reading performance (no locks are used, visibility is obtained via volatility).
    Drawbacks – very bad write performance on large maps.
    Conclusion – use for seldom mutating collections.
  4. Concurrent Collections
    If you want to go heavyweight, consider using: ConcurrentHashMap (or one of its package friends).
    Once you create an iterator over a ConcurrentHashMap, it does not freeze the collection for traversal, updates to the collection may or may not appear during the traversal (weakly consistent).

The approach I ended up taking:
My use case was populating an almost never changing ~ten items cache. A copyonwrite map was the best choice, I believe.

Best pic idea I could think of to visualize Threads :)

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NAT in VMWare vSphere/ESX – In a nut shell

Monday, February 1st, 2010

This post is about NATing an ESX VM, but first, why do I need NAT:

The SIP protocol is not NAT oblivious. To traverse NAT our application has to replace the DNS in the SIP message contact header to the external FQDN that the message receiver will be sending responses to (A NAT with static routing configured).
Therefore I needed to test our software in a NAT topology.

In the past, when we used VMWare player/workstation, it had a build-in NAT network. But, unfortunately, the ESX hypervisor does not provide a NATed network option.
Seeking alternatives at VMWare’s appliance marketplace, I found and downloaded the Vyatta’s community edition (VC5) router appliance (also downladble from sourceforge), and comes under the GPL license.
After 3-4 hours – guided by the official quick start guide -  I had a working NAT configuration in the ESX. Hurray!
Overall, not a hard nut to crack ;) , though I wish VMWare will wise up and just add an build-in NAT option to vSphere.

Left to do:
Obtain some static IPs, so the config won’t break each time the vm reboots and the DHCP lease expires.
Tip #1:
If you want want to access your NATed VM by RDP/VNC, without setting up extra NAT routing rules, consider adding the VM an additional un-NATed NIC, but when doing so, make sure that the OS routing tables are set to route through the NIC that is NATed.
Tip #2:
This short vyatta user installation report also helped me a bit.

Here’s the complete configuration script I ended up feeding to the appliance console (network topology is similar to the one presented in the Vyatta’s getting stated guide):
Where:
1.2.3.4 is your department’s DNS server
192.168.1.199 is the VMs NATed private IP address (provided by the DHCP).
The script contains a NAT forward rule for VNC (port 5900)


configure
set system host-name vyatta-nat
set interfaces ethernet eth0 address dhcp
set service ssh
set service https
commit;
save;
# restart the appliance to switch from console remote desktop to SSH:

#login with user and password
configure
show interfaces

set interfaces ethernet eth1 address 192.168.1.254/24

commit;

delete service dhcp-server
set service dhcp-server shared-network-name ETH1_POOL subnet 192.168.1.0/24 start 192.168.1.100 stop 192.168.1.199
set service dhcp-server shared-network-name ETH1_POOL subnet 192.168.1.0/24 default-router 192.168.1.254
set service dhcp-server shared-network-name ETH1_POOL subnet 192.168.1.0/24 dns-server 1.2.3.4
commit;
show service dhcp-server

set service nat rule 1 source address 192.168.1.0/24
set service nat rule 1 outbound-interface eth0
set service nat rule 1 type masquerade
commit;
show service nat
save;
exit
show nat rules
configure
set service nat rule 20 type destination
set service nat rule 20 inbound-interface eth0
# use a negative fake address to so that all incoming communication will be nated
#set service nat rule 20 destination address !192.168.50.0
#Forward traffic to address 192.168.1.199
set service nat rule 20 inside-address address 192.168.1.199
set service nat rule 20 protocol tcp
set service nat rule 20 destination port 5900
commit;
save;
exit

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New Java blog out there

Monday, January 4th, 2010

A new baby blog was born: Java Tech Sharing.
Proud father: Guy Moshkovich.

I recommend adding to your RSS/Atom reader.

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Utility Frenzy #1 – The log summarizer

Monday, October 19th, 2009

Here’s a post I wrote (in the Hebrew language) which tells the story of the log summarizer utility that I’ve wrote. This story is the first in a line of “utilities stories” I’m planning on writing.
My apologies for those of you whom won’t be able to read it. Posts in this site do appear in English..

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Google is pregnant again – Noop

Monday, October 5th, 2009

After the zillion new dynamic languages that had flooded the earth
(groovy, ruby, …), Google is concocting Noop; a new type-safe language to join Java, Scala, and the rest.

The new language sets out to excel in testability, dependency injection, and readable code (see the proposed features). More interesting than whether Noop will gain a crowd of enthusiasts, is the language’s dynamic and lucid development process; made available through Google code (Sun’s JSRs, are also transparent but here it’s a whole new language).

What do you think?

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Why is Thread.sleep() inherently inaccurate

Sunday, August 23rd, 2009

Avi Ribchinsky, a friend and a college of mien, is transitioning from C++ to the Java world. He had been playing with Thread.sleep(), when he noticed that the sleep method might oversleep more than ordered, and moreover, it could also under sleep (see Fig 1). Coming from the C++ world, that surely caught him surprised ;)

Fig 1.

Thread.sleep() under sleeping

Thread.sleep() under sleeping

How is sleep implemented in Java anyway?

Avi came asking me if I knew anything about it, I was wondering myself how such a common and important method could be faking in the way shown above. Is it the OS? a Bug in the specific JRE version used? Maybe the API doesn’t guarantee milliseconds precision to begin with?
Thinking about all of these factors, we realized that we don’t really know how the JVM implements the sleep method functionality, my best guess would have been that the process registers itself in the OS for a wake up call, and the OS wakes the process via a software interrupt. OK, time to search the web.

The following article gives a very detailed answer, explaining that sleep is implemented by a thread giving up its OS scheduling quantum back to the scheduler, on the next execution quantum the thread gets, it has the chance to wake up and continue processing, or again continue sleeping.
Therefore, the accuracy resolution of sleep is directly dependent on the process scheduling resolution of the operating system in usage. Since windows XP process scheduling resolution is roughly 10ms, the sleep mechanism, in the Avi’s example, might had prefered to under sleep “a little” rather than oversleeping “a lot”, by waking himself in the current scheduling cycle quantum, rather than in the next, future, quantum.

The article also mentions that the inaccuracies are worsened when a process with a higher scheduling priority, than the sleeping process, is in a runnable state.

I assume that, running on a Hypervisor with course grained process scheduling would also produce greater inaccuracies.

sleeping

Conclusion

You can’t rely on the millisecond accuracy of the sleep method. Take a before and after time measurament to find the actual time spent sleeping, in order to avoid ever increasing inacurracies.
Sleep tight :)

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Posted in Multi Threading, Operating Systems, Uncategorized, hardware, performance | 2 Comments »

ESX Server tuning – quick tour

Monday, July 27th, 2009
esx

esx

Our VMWare ESX server does us a great job.
Running on an IBM X3650 HW, with 24GB RAM and 2×4 cores, it can simultaneously run up to 25 virtual machines, each VM is configured with around ~1.5 GB of RAM.

After reaching  the 25 running VMs mark, we started noticing increasing sluggishness when additional VMs were turned on.

Of course, we did the trivial stuff of making sure that all screen savers are disabled, antivirus agents are not correlated to run at the same point in time, and making sure that all of the VMs are running the latest VMWare tools agent.
It was time to dig in deeper to find out where is the bottleneck we came across.

SLKNB_ASomeone told me that the stats that the reliability of the performance indicators that the graphic VI console shows is questionable and it’s recommended using the terminal utilities.So, I SHHed to the service console VM and ran the top utility. Immediately, I understood that what I’m actually doing is surveying the service console VM processes, rather than the overall ESX hypervisor activity. A quick dig up made me realize that the hypervisor is visible through the esxtop command, which is also executed from within the service console VM.

even for those of you that knows your way through the output of top and linux’s sysstat package, the data shown by esxtop is rather cryptic.
This great esxtop tutorial did me a great service with understanding the esxtop output.

I started more than 30 machines to reproduce the problem, and quickly went through the list of usual suspects: CPU, memory and IO:

  • CPU
    I’ve verified that it’s not a CPU problem since the “CPU load average” was around 0.2. and PCPU was much the same.
  • Memory
    Then I’ve switched to the memory display and verified that it’s not a physical memory issue. I saw the “high state” marker which was a good sign + there were almost 17GB ursvd (unreserved memory) in the VMKMEM/MB line.
    SWAP (~3GB) seemed OK.
    VMWare’s ballooning and memory sharing does miracles in broad day light.
  • I/O
    I didn’t see any queues forming. read/write rates seemed pretty low.

So, the 25 VMs performance limit will remain a mystery until I’ll have proper time to analyze it more throughly, or even better, I’ll find someone from IT to do that for me.

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My first question at Stackoverflow.com

Friday, June 12th, 2009

Could stackoverflow.com, or any other programming Q&A service, be the alternative for a serious think process, in which you just put in your question and immediately granted with the perfect answer? Hopefully it is.

To test that I’ve submitted the following “how to regulate the amount of logging printouts” question. Let’s wait, pray, and see if I get any smart/unpredicted answer from any of the 6 billion inhabitant of planet Earth.

question-mark

question-mark

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Increasing the site’s posting rate – new paradigm

Tuesday, January 20th, 2009

I’ve been promising myself to post and publish much more frequent than the current rate of publishing.

Except from reserving more time for the actual posts authoring. I’m also counting on changing the nature of the post as a primary means of increasing the posts rate to once a week or more. From now on, I’ll publish stories from my day-to-day work as a software developer, interesting technical things I come across, questions that I don’t always have an answer for, general discussion, etc. The post will be less educative, less articles like, less accurate, with less checked facts, but on the other hand, much more real life related, much more up-to-date, and presented in an open discussion inviting format.

So, I’ll be writting to you soon as promised :)

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Getting, finally, started…

Thursday, February 28th, 2008

28/Feb/2008 Bootstrap_Blogger Thread#0: Hello! we got ourselves an original site logo (the credit goes to Eliran Nachum). And I’ve got several half completed draft articles that I’m planning on publishing soon enough. So, keep your eyes open ;-)

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