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Re: Moving /tmp to tmpfs makes it useless

On Fri, Jun 01, 2012 at 11:19:40PM +0200, Carlos Alberto Lopez Perez wrote:
> On 01/06/12 13:33, Goswin von Brederlow wrote:
> >> > I don't know the ultimate reason behind this ugly behaviour of Linux
> >> > when the swapping process is happening, but I know this is real and it
> >> > happens because I have experimented this situation myself more than a
> >> > couple of times.
> > It's a matter of that gets swapped and linux choosing the wrong thing to
> > swap (far too often). There is some "bug" in linux that when you have
> > lots and lots of IO at some point the swap heuristics tip over and start
> > swapping apps and usefull data to create more cache space for
> > IO. Doesn't matter that you already have 3GB for cache, it still swaps
> > out your mouse cursor and then things go real bad.
> This makes sense. Many times I have experimented this situation while
> copying a large file from a quick device (hdd) to a very slow device
> (cheap usb stick)
> IMHO The logical way of behaving in such situation is to slow-down the
> IO bandwidth of the processes that are filling the cache, by sending to
> sleep any process that requests more IO while the cache is full instead
> of trying to free RAM by swapping out things from the RAM to the swap.
> Do you know any way to avoid (or mitigate) this? Perhaps some sysctl
> variable?
> Can't Linux be configured to just block (sleep) any process that
> requests IO while the cache is full?
> Perhaps a good idea would be to limit the cache size on a per-PID basis
> and block (sleep) the pid while its cache is full.

I don't think it makes any sense to have a hard per-process limit.
Also, it's not generally possible to account dirty pages to specific
processes exactly.  But I think you will be pleased with this change
that was included in Linux 3.2:



Ben Hutchings
We get into the habit of living before acquiring the habit of thinking.
                                                              - Albert Camus

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