Memory overcommit (was Magic SysRq Key Sequences over Remote DRAC5 Connection)

Jason Edgecombe jason at rampaginggeek.com
Wed Jun 2 18:30:40 CDT 2010


Bryce Nesbitt wrote:
> I'm looking for advice on using Magic SysRq Key Sequences over DRAC5 ssh(see
> http://en.wikipedia.org/wiki/Magic_SysRq_key for background reading on the
> subject).
>
> We have Dell 1950 and 2950 servers with DRAC version 1.20 firmware, and
> normally use putty or OpenSSH to connect.  As far as well can tell the
> serial 'break' does not equate to the SysRq key as documented.
>
> These servers run Debian Linux, and we periodically have problems with out
> of control memory situations.  We could save servers from a hard reboot by
> sending Magic SysReq commands such as "m" and "f" (to dump Linux Kernel
> memory stats and invoke the Out of Memory Killer (oom-killer).  Or make a
> reboot nicer by syncing disks first.  These commands work fine directly from
> an attached keyboard (e.g. Alt-SysRq-m).  Can we get Magic SysRq over DRAC5

Hello Bryce,

I don't have an answer for your sysrq question, but I do have a 
suggestion for your OOM condition.

I suggest tweaking the overcommit_memory and overcommit_ratio kernel 
settings:

See the following links for details:
    http://www.redhat.com/magazine/001nov04/features/vm/
    http://www.linuxinsight.com/proc_sys_vm_overcommit_memory.html

Here are the commands that I use to tune the kernel's memory handling:
    # only allocate based on the over-commit ratio
    /bin/echo 2 > /proc/sys/vm/overcommit_memory

    # only allocate memory if the used ram is less than swap + 90% of 
physical RAM
    /bin/echo 90 > /proc/sys/vm/overcommit_ratio


You might need more swap when using these settings, but you may run 
better. These above settings will cause your application to be denied 
memory when there isn't any. Personally, I prefer that a single 
application fail loudly to halting the whole server and having no clue 
what caused the issue.

Sincerely,
Jason



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