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From Juha Laiho <Juha.La...@iki.fi>
Subject Re: Setting the Right Amount of Memory
Date Sun, 20 Jun 2010 16:02:47 GMT
On 20.6.2010 14:06, Mark Thomas wrote:
> On 20/06/2010 00:30, André Warnier wrote:
>> Just a note here : 160 X 512 MB = 81 GB.
>> If each Tomcat's JVM is allowed to use up to 512 MB of Heap, there might
>> be moments where a lot of JVM's will be using close to that amount.
>> Unless your system can really support that amount of real RAM, you may
>> be in for massive swapping.
>
> And that could be a major problem. JVMs and swapping do not place nicely
> together. To quote some folks at work that have been looking at this
> recently performance "falls of a cliff".

Yep, I've seen that as well -- and the effect really is easy to
understand, esp. in the light of how Chuck explained the
mark-sweep-compact memory management.

The live objects are somewhat scattered throughout the heap, and
walking through the live object graph will touch all of them -- thus
if any was paged out at the moment, it will be paged in (most likely
forcing some other page out). If majority of the OS-level virtual
memory consumption consists of JVM heaps, then it's very likely that
the paged-out memory page does contain a live Java object - which will
again be paged in when the owning JVM does its "mark" phase.

Paging is suitable OS-level memory management for processes with full
pages of data that only needs to be accessed infrequently (or perhaps 
only in the initialization phase of the application). Then these 
seldom-accessed pages can be moved out to disk, freeing RAM for more
frequently needed items. With JVM memory management, everything is
accessed every now and then, making paging a real performance-killer.

-- 
..Juha

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