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Old 12-19-2011, 02:37 AM   #1
dezavu
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what is memory pressure?


Hi,

In Enterprise Linux what is memory pressure? & is there any command that would show the system to be in such a state?



Thanks & Regards.
Vijay
 
Old 12-19-2011, 07:21 AM   #2
tronayne
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Memory pressure is a "metric" (don't you just love those buzz words?).

Essentially, say you've got a bunch of virtual machines on a given server. You've got, oh, 16G RAM in the box. You allocate maybe 4G RAM to each virtual machine. You start up two of 'em (so 8G RAM is used by the virtual machines and you've got 8G RAM for the server operation system); no pressure there. Start another and you're up to 12G virtual and 4G system, still not too much. Start another and the whole thing comes crashing down (because the system will be swapping like mad and things grind quickly to a halt). That's memory pressure -- you allocate too much memory to too many things and you spend all your time swapping (we used to call that trashing).

The lesson is that you need more RAM to support more virtual machines (or more applications).

Memory pressure is reported by nworks, http://www.nworks.com/, and appears to be useful with VMware. Take a look at the link for more information.

But, you can save yourself a lot of trouble by simply figuring out what you need to support what you're doing and make sure there's sufficient capacity to do so -- things like enough RAM (number one) and spreading the load over multiple servers.

Hope this helps some.
 
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Old 12-19-2011, 08:29 AM   #3
johnsfine
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I have always seen "memory pressure" used as an abstract quantity that is important for understanding the performance of a computer system, but is hard to exactly define and harder to measure.

It relates most closely to virtual memory fault rates, the rates at which processes try to access pages that are logically mapped in their address space, but physically not mapped. Those may be "soft faults" (the required page is in the cache and/or resident in some other process's physical mappings) or "hard faults" (the required page must be brought in from disk).
 
Old 12-23-2011, 11:34 AM   #4
markseger
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Kind of hard to answer in the right context, but as another example is that when you read/write a file its blocks typically pass through a system cache and stay there until the file is deleted. This is why you often see almost no free memory which can freak out a lot of people. The one caveat to this is when someone else needs to do I/O and hence needs room in the cache. The act of telling the operating system to free up some of the cache for use by another process is called 'memory pressure' and when this occurs the o/s determines which pages are felt to be stale enough to get rid of, thereby freeing up more memory.
-mark
 
  


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