Re: [dm-devel] [Lsf-pc] [LSF/MM TOPIC] a few storage topics

["Loke, Chetan" <Chetan Loke netscout com>]

> 
> > So there are two separate problems mentioned here.  The first is to
> > ensure that readahead (RA) pages are treated as more disposable than
> > accessed pages under memory pressure and then to derive a statistic for
> > futile RA (those pages that were read in but never accessed).
> >
> > The first sounds really like its an LRU thing rather than adding yet
> > another page flag.  We need a position in the LRU list for never
> > accessed ... that way they're first to be evicted as memory pressure
> > rises.
> >
> > The second is you can derive this futile readahead statistic from the
> > LRU position of unaccessed pages ... you could keep this globally.
> >
> > Now the problem: if you trash all unaccessed RA pages first, you end up
> > with the situation of say playing a movie under moderate memory
> > pressure that we do RA, then trash the RA page then have to re-read to display
> > to the user resulting in an undesirable uptick in read I/O.
> >


James - now that I'm thinking about it. I think the movie should be fine because when we calculate the read-hit from RA'd pages, the movie RA blocks will get a good hit-ratio and hence it's RA'd blocks won't be touched. But then we might need to track the hit-ratio at the RA-block(?) level.

Chetan