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From Emmanuel LŽcharny <elecha...@gmail.com>
Subject Re: DIRMINA-391 vs DIRMINA-576
Date Tue, 26 Jan 2010 15:55:54 GMT
Dan Lawesson a écrit :
> Hi,
> Until yesterday I was under the impression that heap buffers are the 
> way to go and direct buffers are more of a relic of at most historical 
> value. This was a belief that was at least partly supported by the 
> Apache MINA project documentation of the upcoming version 2.0. 
> Yesterday my view on heap buffers was completely reversed when I 
> witnessed a perfectly working legacy system (Java 1.4.2 something) 
> barf and die due to lack of direct memory even though we did not 
> explicitly use any direct heap buffers at all. Post mortem heap dumps 
> show that there had been a creation of 100 direct buffers per second 
> and none of them were cleaned since heap memory was fine and no full 
> GC sweep was triggered. The heap contained a chain of 200k cleaners 
> after 30 minutes of uptime. The reason I have not witnessed this 
> before is probably that usually these systems also use a lot of heap 
> memory and therefore trigger full GC which triggers the cleaners.
> After a while we realized that in case you use heap buffers, you get a 
> temporary direct buffer. Clearly, at least in this old JVM these 
> temporary direct buffers are not even pooled (that was the WTF of the 
> day!).
> This lead me to DIRMINA-391 
> http://issues.apache.org/jira/browse/DIRMINA-391. Trustin Lee 
> advocates heap buffers, even though the issue presents a strong case 
> against them. Does this mean that this behavior of using non-pooled 
> temporary direct buffers is a thing of the past, i.e., more modern 
> JVMs are smarter than that? This is the only explanation I can find to 
> why MINA 2.0 uses heap buffers per default and also has dropped the 
> pooling feature of older MINA versions.
> The reason I write this mail is that both 
> https://issues.apache.org/jira/browse/DIRMINA-576 and the linked Sun 
> issue seem to contradict this. They both make a strong case for always 
> using pooled direct buffers. What is the community consensus on heap 
> buffers?
Concensus, I don't know. My strong opinion is that direct buffers are to 
be used when dealing with low level IO, and heap buffers must be used 
for everything else.

The fact that you can kill your OS because you eat all of it's memory 
seems to me a big no-no for DirectBuffer. If I use Heap buffers and if I 
suck up all my JVM memory, too bad for me, my code and my salary, but at 
least, it impacts only my application's JVM.

In your case, the Direct buffers are used to exchange data with a socket 
in a IOread, done by the JVM : makes perfect sense. The question is why 
aren't they cleaned ?

I have to dig back into those issues I guess...

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