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From James Golick <jamesgol...@gmail.com>
Subject Re: Instability and memory problems
Date Sun, 20 Jun 2010 19:58:02 GMT
uh. wow. I just read up on all this again, and read the code, and I'm a
little surprised, to be honest.

There's no attempt to manage the total size of the mmap()'d IO, and the
default buffer allocation is quite sizeable. So, basically, if you have any
data, over time, you will run out of memory, and there's no way at all to
control it.

Can we consider changing the default?

On Sun, Jun 20, 2010 at 3:37 PM, James Golick <jamesgolick@gmail.com> wrote:

> Thanks for your thoughts. Answers below:
>
> On Sun, Jun 20, 2010 at 2:21 PM, Peter Schuller <
> peter.schuller@infidyne.com> wrote:
>
>> > The memory problems I've posted about before have gotten much worse and
>> our
>> > nodes are becoming incredibly slow/unusable every 24 hours or so.
>> Basically,
>> > the JVM reports that only 14GB is committed, but the RSS of the process
>> is
>> > 22GB, and cassandra is completely unresponsive, but still having
>> requests
>> > routed to it internally, so it completely destroys performance.
>> > I'm at a loss for how to diagnose this issue.
>>
>> Sorry, I don't know the history of this (you mentioned you've alluded
>> to the problems before), so maybe I am being redundant or missing
>> something, but:
>>
>> (1) Is the machine swapping? (Actively swapping in/out as reported by
>> e.g. vmstat)
>>
>
> Yes, somewhat, although swappiness is set to 0.
>
>
>> (2) Do the logs indicate that GC is running excessively, thus
>> indicating an almost-out-of-heap condition?
>>
>
> It runs, but I wouldn't say excessively.
>
>
>> (3) mmap():ed memory that is currently resident will count towards
>> RSS; if you're using mmap():ed I/O (the default), that is to be
>> expected.
>>
>
> This is where I'm a little confused. I thought that mmap()'d IO didn't
> actually allocate memory. I thought it was just IO through a faster code
> path.
>
>
>> (4) If you are using mmap():ed I/O, that is also in and of itself
>> something which can cause trouble if the operating system decides to
>> swap your application out in favor of the mmap()
>
> (5) If you are swapping (see (1)), try switching from mmap():ed to
>> standard I/O (due to (4)), and/or try decreasing the swappyness if
>> you're on Linux (see /proc/sys/vm/swappiness).
>>
>
> I tried switching to standard IO mode, but it was very, very slow. What I'm
> confused about here is that if mmap()'d IO actually allocates memory that
> can put pressure on other processes' memory, is there no way to bound that?
> If not, how can anybody safely use mmap()'d IO on the JVM without risking
> pushing their process's important pages out of memory.
>
> swappiness is already at 0.
>
>
>> (6) Is Cassandra CPU bound or disk bound in general, regardless of
>> swapping?
>>
>
> Hard to tell because of the paging.
>
>
>>
>> --
>> / Peter Schuller
>>
>
>

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