Memory Buffers

Aaron Smith asmith at nexcerpt.com
Thu Nov 7 09:31:52 CST 2002


	Ok.  I have, what would seem to be, a fairly simple question.  When I
cat out the contents of /proc/meminfo on my Poweredge 4400, there is a
value for "Buffers".  What is this memory used for?  After doing some
research, all I've found is that they are I/O Buffers set aside by the
kernel.  My assumption, then, is that disk I/O is buffered into this
chunk of memory before being written out to disk.  Is this correct?
	The REASON for the question is that our Poweredge 4400's (running
Redhat 7.1 and a non-rpm based 2.4.14 kernel) will suffer a massive
spike in CPU load (25-30 is the average) whenever this buffer value
drops to below 2Mb.  The machines have 4GB of RAM, and there is
typically around 3GB of that set aside in the page Cache.  The Buffers
value fluctuates up and down quite a bit, but the mean average has been
slowly decreasing over time.  A few months ago, it would hang out around
100 or 80MB, It slowly started dropping until it hangs out around 30MB
and will bottom out at less than 2MB.
	My understanding is that Cache and Buffers will be freed by the kernel
when a process needs memory.  My assumption, therefore, is that the
dropping of the buffers value means that that memory is being freed up
for running processes.  Why would the kernel free up some of the 30MB of
buffers rather than a portion of the 3GB of Cache?
	Is there a way to tell the kernel to always have X amount of Buffers
available?  The biggest software of note running on these machines is
Postgres Database software and a number of backend processes that use
said database.
-- 
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Aaron Smith             		vox: 269.226.9550 ext.26
Network Director        		fax: 269.349.9076	
Nexcerpt, Inc.          		http://www.nexcerpt.com
	
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