[Date Prev][Date Next] [Thread Prev][Thread Next] [Date Index] [Thread Index]

Re: High Memory Kernels and buffer bouncing -solved ?

I hope, I solved the High Memory problem.

Today, I rebooted to a debian stable 2.4.18 kernel that was patched with
both XFS support and the Andrea Hi-Memory patch.

As a test, I ran an rsync on the mail data during the peak load time to force a
heavy IO load. When I did this before, I would usually get complaints
from users. However in this initial test, there was no noticeable IO
wait. All 3000+ accounts were rsync-ed in an about 2 hours. There was
about 2-4GB of changes since the last rsync. 

Additionally, the CPU load never went above 20%. Prior to getting the proper patch, the
CPU load would occasionally go up to 80-100% due to to what I thought
might be buffer bouncing. If the problem does not return in a week, I
will change this from 'thought might be' part to 'was'.

I call this an initial test because the next test is simply the test of time.

Actually, solving the High Memory problem is hard because it does not
come up very often. It only occurs during heavy IO times. The last time
my problem came up was during a weekly cronjob run. 

Thus, if you have a server with a lot of RAM (over 4GB with High-memory
support compiled in) that has a very high IO load like a mail-server or a
web-server, and your system stats tell you that you are running out of
RAM occasionally, then you might have a bounce buffer problem. 

To resolve High Memory issues you will have to patch your Kernel with
the Andrea patch or use the Red-Hat Enterprise kernel as R.C.

        Red Hat creates kernels with all the needed patches for the
	        high-memory stuff. You just have to switch them on in the
		kernel on the Debian side this is not always guaranteed.

	The high memory patch from andrea:


On 04/12/03 11:52 -0500, Theodore Knab wrote:
> I was wondering if anyone is running a heavy worked server with memory above 4GB on a 
> SMP 32 bit i386 machine.
> I think that my mail server at work is getting bounce buffers, but I am not sure 
> how to detect them. Does anybody no how ?
> I compiled my kernel for High Memory support, but I am not sure this is
> all I need to do. Is there another patch I need to worry about or should
> a stock Debian kernel provide me with all the needs for High memory.
> There is a person on http://kernel.org named  Matthias Andree that wrote a
> highmem patch and was wondering if that was in the Debian Kernels ?
> The reason I ask is that occasionally, the server performance spirals down
> exponentially when both the IO and users logged in gets to a certain
> level. This seems too look like the buffers are bouncing on the memory
> channel as documented here:
> http://lkml.org/lkml/2003/9/17/81
Ted Knab
Chester, MD 21619


Reply to: