Hi,

Jukka Larja (roskakori2006.delete@this.gmail.com) on January 23, 2020 6:36 am wrote:
> Brendan (btrotter.delete@this.gmail.com) on January 22, 2020 3:28 pm wrote:
> > Jukka Larja (roskakori2006.delete@this.gmail.com) on January 22, 2020 6:12 am wrote:
> > > Brendan (btrotter.delete@this.gmail.com) on January 22, 2020 3:32 am wrote:
> > >
> > > > The right way to think about the way memory management works in Windows (unless swap is
> > > > disabled by an idiot) is to think of physical memory as nothing more than a cache of the
> > > > page file; where "max. virtual memory size = page file size". Physical memory size and
> > > > physical memory availability are irrelevant - they have no effect on OOM whatsoever.
> > >
> > > No, that's not right (you may be right for some older Windows version, but I'm pretty sure even
> > > 2000 didn't work like that). By default, Windows allocates very small page file, especially on a
> > > system with large RAM. Like I wrote in the other post, on 64 GB system, page file was 9 GB[1].
> >
> > OK; my description was a little incomplete, and could've been more like "think of physical memory
> > as nothing more than a cache of the page file; where "max. virtual memory size = page file size",
> > and where "page file size" may not be a fixed size and may grow as needed (if possible)."
> >
> > I probably should've also said "swap size" instead of "page file size" to cover non-default configurations.
>
> Still wrong. Windows is quite happy to run with page file significantly smaller than physical memory
> (running completely without page file may be stupid for unrelated reasons, of which I'm not that
> familiar with). That's probably the normal mode of operation with current memory amounts.

Why wrong?

I'd expect that with page file significantly smaller than physical memory Windows would only commit to "max. swap"; and then the rest of the physical memory (that can't be used for "cache of swap") would be used for "cache of file data" (and anything else Windows might cache that is not part of "memory processes committed to").

Note: All files (and parts of files) mapped as "read-only" wouldn't be included in "how much OS has committed to" (those area can be backed by file system not swap).

> > For example, I'm not sure if Windows
> > can be configured for multiple swap providers in a sensible way (e.g. a smaller swap
> > partition on SSD with higher priority plus a larger swap partition on a slow "rotating
> > metal" drive with lower priority, with no "auto-adjust page file size").
>
> I don't know if Windows does anything sensible regarding storage performance with multiple
> page files, but you can configure one per filesystem, either an automatically managed
> or manually managed (minimum and maximum size). Swap partitions don't really make sense
> in Windows, as you'll get constantly spammed by low diskspace warnings for them.
>
> I'm sure there are all sorts of non-normal-user-accessible-options to tune all that.

Oh my - I did some checking and it seems that Windows doesn't even support swap partitions (it only supports "page file on normal file system"); so it always has to worry about normal files and swap competing for the same space, can't avoid "file system meta-data" overhead, and has to worry about file fragmentation for the page file.

For "tiered swap", even if it is supported I don't think any well known OS will (e.g.) move "least recently used data" from faster swap to slower swap so that the faster swap can be used for "less recently used data". E.g. for Linux (if it's configured for multiple swaps with different priorities) I think the priorities only effect allocations, so (under some conditions) you might end up with faster swap full of "least likely to be used" and slow swap being pounded because faster swap is full; which isn't great for some configurations (e.g. "zram as 1st level swap, slow disk as 2nd level swap", to achieve some of the benefits of the "memory compression" feature on OS X).

For Windows, in theory it might support "tiered swap" (e.g. by benchmarking swap providers in a "zero configuration, auto-detection" way); but it's easier to assume Windows doesn't support it at all (just treats all swap providers as equal).

- Brendan