Posts tagged windows services

Thread.sleep() under sleeping

Why is Thread.sleep() inherently inaccurate

Aug 23rd

Posted by Gili Nachum in general

2 comments

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 preferred 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 :)

, , , , , , , ,

Miniature scale pro-active IT – Creating windows services dependencies

Sep 4th

Posted by Gili Nachum in Operating Systems

1 comment

Yesterday night we had a scheduled power shutdown in the Dev lab. Today morning, the lab manager rose up early to get all servers running before the armies of developers arrive to the office. My work includes interacting with a Lotus Sametime server (IBM’s Instant Messaging (IM) server), so I run my own private IM server. starting the day’s work, I quickly noticed that my IM client fail to log in to the IM server. In fact, all of the developers could not log in to their own private IM servers.
Remembering that during the client log in process the IM server validates the client supplied log in credentials against a central LDAP server, the LDAP server became an immediate suspect.

The LDAP server we’re using is an old IBM ITDS LDAP server running on Win2000. It’s comprised out of two processes: the ITDS process that parses and execute LDAP queries, and a DB process (DB2) that takes care of data persistency. Both processes are registered as Windows services.

The investigation commenced! Maybe the LDAP server is down? I went a head and checked the ITDS and DB2 services status, both were running. Hmm… I moved on to inspect the ITDS log, and saw that during its start up stage it failed to create a connection to the DB2, therefore it resorted to starting in a, “crippled”, configuration only mode. That means that it just sits there, wasting random CPU cycles, giving the illusion that it’s there to provide service, but not actually answering any queries.
To remedy the situation I simply re-started the ITDS service. It started up normally and began servicing incoming LDAP queries from the IM servers.
At this point, you’ll be tempted to announce world wide: “I fixed it!”, but before you do that, stop and think about it for a minute; what is it exactly that you fixed? In did, the ITDS began servicing queries, and the client can log in, meaning that the current manifestation of the problem was eliminated, but did you fix the problem itself? Part of being pro-active means that you solve future problems before they actually occur. What stops the problem from re-occurring the next time someone decides to restart the server? in order to solve it for good, you first need to understand what was the cause of the problem.

So, what happens during a server start up? The ITDS and DB2 services startup-type is set on Automatic, thus they start when the OS starts. The db connection error message fits a scenario in which the ITDS service started and tried to connect to the DB2 before the DB2 service finished starting up.
We would like to instruct the ITDS service to be less hasty, and wait for the DB2 service to finish starting, before stepping into itself start up process. Educating it can be achieved by defining a service dependency, stating that the ITDS service is dependent on the DB2 service.

Implementing it: Dependencies can’t be created using the windows MMC “computer manager” snap-in GUI, so you’ll have to get your hands dirty with registry mud using the following procedure.

Problem uprooted! You won you pro-activity badge.

, , , ,