ZFS ARC under memory pressure

Konstantin Belousov kostikbel at gmail.com
Sun Aug 21 08:28:53 UTC 2016


On Sat, Aug 20, 2016 at 11:08:44AM -0500, Karl Denninger wrote:
> If you are talking about the case of an executable in which part of the
> text is evicted you are correct, however, you are still choosing in that
> instance to evict a page for which there will likely be a future demand
> and thus require an I/O (should that executable come back up for
> execution) as opposed to one for which you have no idea how likely
> demand for same will be (a data page in the ARC.)
No, I am not talking about only text segments.
Any clean page can be reused after unmapping.

> 
> Since the VM has no means of "coloring" the ARC (as it is opaque other
> than the consumption of system memory to the VM) as to how "useful"
> (e.g. how often used, etc) a particular data item in the ARC is, it has
> no information available on which to decide.  However, the fact that an
> executing process is in some sort of waiting state still likely trumps
> an ARC data page in terms of likelihood of future access.
Buffer cache behaves exactly the same, since access references are not
counted for the pages constituing buffers.


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