Re: [RFC] 2.3/4 VM queues idea

[Jamie Lokier <lk@tantalophile.demon.co.uk>]

Matthew Dillon wrote:
>     Virtual
>     page scanning has severe scaleability problems over physical page
>     scanning.  For example, what happens when you have an oracle database
>     running with a hundred independant (non-threaded) processes mapping
>     300MB+ of shared memory?

Actually you can make this scalable quite easily.   I think it's
asymptotically equivalent to physical page scanning.

First, ensure the async. unmapper can limit the number of mapped
ptes.  Method: whenever the number of established ptes increases
above a high water mark (e.g. due to a page fault), invoke the unmapper
synchronously to push the number below a low water mark.  (Both marks
can be the same).

Second, make the scanner scale independently of the virtual addresses
used.  Method: store boundary tags in the /unused/ ptes so that
scanning skips unused ptes.  Ok, this can have fiddly interactions with
not-present swap entries.

In this way, the work required to scan _all_ mapped pages can be
strictly bounded.

cheers,
-- Jamie
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