Date: Thu, 15 Sep 2011 11:36:59 -0400 From: Jonathan Vomacka <juvix88@gmail.com> To: Matthew Seaman <m.seaman@infracaninophile.co.uk> Cc: Michael Sierchio <kudzu@tenebras.com>, freebsd-questions@freebsd.org Subject: Re: Recommended SWAP space for large amounts of ram (8GB) Message-ID: <4E721B9B.2020300@gmail.com> In-Reply-To: <4E70F79A.3090806@infracaninophile.co.uk> References: <4E709F62.60705@gmail.com> <4E70B269.4020900@infracaninophile.co.uk> <CAHu1Y71Yt9ra=aBGP519EZvCR4=HMFK_%2Bv79X_awjZ%2BsaZXTtw@mail.gmail.com> <4E70F79A.3090806@infracaninophile.co.uk>
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Thanks Matthew / Michael for your responses on this. On 9/14/2011 2:51 PM, Matthew Seaman wrote: > On 14/09/2011 18:27, Michael Sierchio wrote: >> On Wed, Sep 14, 2011 at 6:55 AM, Matthew Seaman >> <m.seaman@infracaninophile.co.uk> wrote: >> >>>> ... In these days of plentiful RAM, the new rule of thumb is "if you're >>>> swapping, then you're doing it wrong." > >> I think your response follows the excellent pedagogical principle: "a >> little inaccuracy saves a lot of explanation." But... disk is still >> (by far) the cheapest commodity, and the opportunistic paging >> algorithm manages VM very well. VM is not by any means obsolete, and >> seeing paging behavior is not a sign of a misconfigured system. > > Well, yes. I was certainly glossing over a lot of complexity -- but I > would maintain that I am fundamentally correct. > > Having some pages swapped out is absolutely not a problem. True. In > fact, it's a positive benefit: swapping out memory pages that are > exceedingly rarely referenced makes more room in RAM for more actively > used pages. > > On the other hand, having pages continually swapping in and out > definitely is a problem in terms of performance, given that disk IO > takes of the order of milliseconds, while reference to main RAM is of > the order of microseconds or less. Orders of magnitude faster. > > Now, while disk may well be the much the cheapest storage medium > available, that's only part of the expense. In fact, up-front capital > expenditure on the kit (perhaps several thousand pounds/euros/dollars) > is outweighed by the operational expense (power, cooling, hardware > support etc.) over the life of the equipment, so spending a bit more > (capex) on components that run at lower power (opex) makes a lot of > sense. Even more, if the server is being used for eg. e-Commerce, then > the volume of the transactions and the data processed by the server > makes all the difference to your margin: the more you can do with the > same hardware - viz, the more efficiently and faster you can make the > hardware run - then the more profit you make. Buying more RAM is > peanuts on that scale. > > Cheers, > > Matthew >
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