Re: [gentoo-user] mysqld invoked oom-killer

[Dale]

Dale wrote:
> Grant wrote:
>>>> Then why not have a really big swap file?  If swap is useful as a
>>>> second layer of caching behind RAM, why doesn't everyone with some
>>>> extra hard drive space have a 100GB swap file?
>>>>
>>> You've not understood what I said, I think.  Swap is not useful as
>>> filesystem cache.  Swap is as efficient (probably a little less) than
>>> the files on the disk.  It's RAM that's efficient as filesystem cache.
>>>
>>> Where swap comes in is the kernel can swap out pages from "stale"
>>> processes, and reclaim the RAM as filesystem cache.
>> That all makes perfect sense, but if a small swap is good and a large
>> swap is not any better, I'm missing something.  Maybe the pages from
>> stale processes never total more than a small amount?  I don't see how
>> that could be.
>>
>> - Grant
>>
>
> To confuse you even more, there is a swappiness setting as well.  On
> my old x86 rig, I have 2Gbs of ram.  My hard drive is really slow
> since it is IDE.  I set swappiness to 20.  That tells the kernel that
> I have swap space but don't use it unless you must.  For what I use
> the rig for, 2Gbs is plenty of ram.  The lower the swappiness setting,
> the less the kernel will try to use *SWAP* .  The higher the setting,
> the more it will try to use swap.
>
> I have a new rig that is amd64 and has SATA drives which are pretty
> fast.  I still have swappiness set to 20.  Why do I have it set to 20
> when the drives are faster you ask?  I have it set to 20 because I
> have 16Gbs of ram here.  Even if I have portage's work directory on
> tmpfs and am compiling OOo, it should not need swap then either.
>
> By the way, my swap partition is 1Gb on both systems.  Why have it
> this way since one machine has 2Gbs and one has 16Gbs?  As it has been
> said, you want a little swap and even using a little swap is OK.  You
> just don't want it to be using swap and actually swapping data all the
> time.  On my old rig, it started out with 512Mbs.  I use KDE and it
> got to the point where it was using enough ram that it was not just
> using swap and letting things sit, it was actively swapping data from
> swap and doing so a lot.  It would only be using a 100Mbs sometimes
> 200Mbs.  The point is, it was slowing the system down because of the
> swapping process.  I bought a stick of ram and all was well again.  It
> would still use a 100Mbs of swap at times but it would not be actively
> swapping the data back and forth so it wasn't a big deal.
>
> I think the point is this, it is good to have a little swap.  It is
> even OK for it to use a little swap when it is mostly sitting there.
> When you notice it using swap and it is actively swapping and moving
> things back and forth, you need more memory.  Having the swap may can
> save you from a crash but is can also give you a "time to add more
> ram" hint too.  If Linux starts using swap a good bit, you need more ram.
>
> I do like that attic analogy tho.  You may not mind going up in the
> attic and dragging the tree down once a year but you may not want to
> go to the attic to get a glass of water.  That would put a lot of wear
> on the stairs and it would also get old after a while to.
>
> Dale
>
> :-)  :-)
>

P. S.  Corrected a disconnect between brain and keyboard.  It is in 
*bold* in the first paragraph.