[rabbitmq-discuss] Memory High Watermark.

[Chris Nicel]

Hi Simon/Duncan,

Thanks for your feedback. Useful information.

Cheers

Chris

-----Original Message-----
From: Simon MacMullen [mailto:simon at rabbitmq.com]
Sent: 16 October 2013 16:01
To: Discussions about RabbitMQ
Cc: Chris Nicel
Subject: Re: [rabbitmq-discuss] Memory High Watermark.

On 16/10/13 15:43, Chris Nicel wrote:
> I would like to know more about the decision to default the memory
> high watermark to 40% of the installed RAM. Is this designed for use
> on a server with contended resources? If the server is dedicated to
> running RabbitMQ can this be raised to say 80% or 90% safely?

The original reason was a fear that although Erlang has per-process GC, we could theoretically end up in a situation where one process filled most of the VM, and the extra space required to GC it caused the machine to run out of space altogether.

That's quite unlikely though, the typical case is likely to be a VM with no one process dominating.

There's a second reason which I find more compelling: the high watermark is the point at which the broker stops accepting new messages, not the point at which it stops consuming more memory. In theory the one controls the other, but in practice there's likely to be some hysteresis.

We can't assume that swap space exists either, we have to assume that if we run out of memory then that's game over. If like almost everybody you do have swap space working then you can probably raise the limit somewhat. Personally I would be comfortable at 60 or 70%.

Cheers, Simon

--
Simon MacMullen
RabbitMQ, Pivotal

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