On Tue, Sep 14, 2010 at 08:36:40PM -0400, Richard Pieri wrote:
> > Yeah except that's not what gaf meant, which was fairly clear from
> > context, which is why what you said made no sense.  He was talking
> > about memory management, where VM clearly means virtual memory, and
> > virtual machine is not especially interesting or relevant.
> 
> Whereas I was explicitly writing about application memory fragmentation, where VM clearly means virtual machine

Yes, clearly, because obviously only virtual machines experience
memory fragmentation, and virtual memory couldn't possibly be related
in any way.  [Yes, I'm being sarcastic.]  I note that while you quoted
your comment about Java, you didn't quote the message to which your
comment was a reply (which had nothing to do with Java).  You seem to
be confused... You were replying to this comment of Jerry's  about
memory management:

> On Sep 14, 2010, at 11:37 AM, Jerry Feldman wrote:
>> On 09/14/2010 10:58 AM, Richard Pieri wrote:
>>> What Jack said about applications.  Even though an application
>>> doesn't leak memory it can still suffer from memory fragmentation
>>> which will eventually degrade performance.  File servers, on the
>>> other hand, generally don't care.  Quarterly may be overkill, or it
>>> may be woefully inadequate.  It depends on what the server is doing.
>>
>> I disagree with this on modern Linux (and BSD) kernels. Let's
>> remove VM from the discussion for a second. Most applications
>> use the malloc(3)        
>>                                                                              
> Let's not, because I'm specifically thinking of Java's garbage
> collection as a common example.

...where he was clearly talking about virtual memory, and then you
replied  -- in a way which made no sense in context -- about the JVM.
Then when he replied to your confused reply, IN context to his
original point, you castigated him for going off on tangents, when in
actuality it's you who has gone off on a tangent.  Note that I
included your previous comment so that you can see that it also had
nothing to do with Java, and that Jerry's response was perfectly in
line and not a tangent.

> Virtual memory is entirely irrelevant.
 
Not to Jerry's point, which is why he explicitly dismissed it (see
below).

> > Really?  I've never heard of any such issues causing sweeping problems
> > in the sysadmin community...  You'd think that if the Linux kernel
> > had "been *very* vulnerable to memory fragmentation issues" you'd have
> > heard all sorts of reports about wide spread system crashes.  Methinks
> > you overstate the case by rather a lot.
> 
> Memory fragmentation does not result in system crashes.  

Sorry, I meant program crashes, although it could concievably cause a
system crash if the kernel needs to allocate a large contiguous block
for itself and can't due to fragmentation (and it has run out of
virtual memory), though this would require a bug in the kernel's
memory management module or something similar.  But it's unclear to me
why memory fragmentation would cause noticable performance degredation
*except* in the case where the kernel can't allocate large enough
contiguous blocks *and has to page processes out to make room* (Jerry
said this too, BTW).  Which is exactly why virtual memory is NOT
irrelevant, and why Jerry mentioned it explicitly.

The correct solution to *that* problem is to reduce contention for
unused blocks...  Either increase your RAM, or move processes
somewhere else.  It's fine and helpful for the kernel to try harder to
reduce fragmentation, but that ultimately doesn't solve the real
problem, except in mostly marginal cases...  It'll only be a matter of
time before you need to solve the problem the right way.

> Here's some useful information about it

This link is actually much more useful:

  http://serverfault.com/questions/133305/linux-memory-fragmentation

-- 
Derek D. Martin    http://www.pizzashack.org/   GPG Key ID: 0xDFBEAD02
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