system hang ?

kutty_prasad · 12-03-2007, 03:19 AM

Hi All,

I have question regarding swap usage. Recently few of our servers were not responding to any login (rsh,ssh,telnet,etc). After sometime even ping won't respond. The logs are clean and does not show any information related to system hang. My question is, will heavy swapping cause the system to hang. (For example, running a memory hungry application). It yes, please explain why? BTW: We use RHEL4-U4 x86_64 WS.

Thanks.

matthewg42 · 12-03-2007, 03:48 AM

If your machine gets memory starved it might start to perform really badly, to the point where you cannot do very much at all. Extreme memory starvation may lead to thrashing (where all the time is spent paging, and little or no work gets done). As a system administrator, you should monitor your systems and apply limits to processes to prevent this.

[edit]Of course that is a lot easier said than done...[/edit]

syg00 · 12-03-2007, 04:37 AM

Better off looking for evidence than guessing.
RHEL should have sysstat installed - what does sar show ???.

kutty_prasad · 12-04-2007, 02:40 AM

Thanks for the reply Mathew.

Syg00: We have sar installed and I actually found the high mem. usage and swapping from sar output. That's why I wanted to know "why system hangs while heavy swapping?".

Also I feel Solaris is more stable even when there is high swap usage.

Thanks

syg00 · 12-04-2007, 02:55 AM

Good - er, I mean it's good you have hard data, not that you've got a problem ...

Is this a machine that has (recently) migrated from 2.4 to 2.6 ???.
What do the disk drives data look like - in/out rates, wait%, things like that.

Swap usage ain't necessarily bad - lots of swap movement can make a serious mess of I/O response times.
Can't talk to Solaris - never been there.