A few months ago I noticed that Desktop Windows Manager would eat more
and more memory until it noticeable slowed the computer down.

I did a lot of searching for info on that. I found that others were
experiencing the same problem. I found some suggested cures. Nothing
worked.

I discovered myself that I could bypass frequent reboots by using "end
task" in task manager, which only rebooted the Windows Manager. So at
least I didn't have to do repeated system reboots.

Several people here had suggestions, and we left it at Firefox as
being the likely culprit. I was never fully convinced of that, since
the problem did occur occasionally when I had not been using Firefox.

I always suspected it was a Windows-10 problem. Well, now it seems I
was right. A few days ago I installed the latest Cumulative Update for
21H2. Low and behold, problem gone! Desktop Windows manager now never
goes over about 50 mb ram use. More important, after a window is
minimized or closed, the ram used goes DOWN, which it never did
before.

Couldn't MS at least have announced they were aware of the problem and
looking into it?

-dan z-

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On 10/24/2022 8:24 AM, dyno dan wrote:
> A few months ago I noticed that Desktop Windows Manager would eat more
> and more memory until it noticeable slowed the computer down.
>
> I did a lot of searching for info on that. I found that others were
> experiencing the same problem. I found some suggested cures. Nothing
> worked.
>
> I discovered myself that I could bypass frequent reboots by using "end
> task" in task manager, which only rebooted the Windows Manager. So at
> least I didn't have to do repeated system reboots.
>
> Several people here had suggestions, and we left it at Firefox as
> being the likely culprit. I was never fully convinced of that, since
> the problem did occur occasionally when I had not been using Firefox.
>
> I always suspected it was a Windows-10 problem. Well, now it seems I
> was right. A few days ago I installed the latest Cumulative Update for
> 21H2. Low and behold, problem gone! Desktop Windows manager now never
> goes over about 50 mb ram use. More important, after a window is
> minimized or closed, the ram used goes DOWN, which it never did
> before.
>
> Couldn't MS at least have announced they were aware of the problem and
> looking into it?
>
> -dan z-

I suspect part of what DWM does, is manage compositing. This uses
video card memory, but maybe the main memory has a copy too.
(And then the info is pushed to the video card, 60 times a second.)

Years ago, the cross platform estimate for compositing, was
that "128MB should be enough". That's how you knew how big
your video card memory had to be, to stand a chance of
supporting modern compositing. Virtually every popular OS,
supports compositing to some extent. (This is why Firefox
can be cross-platform -- it likely has no option at all
for OSes that don't have compositing.) If you only have
one window open, the usage of that memory is minimal.
Each added window, needs memory to store the pixels.

My interpretation of what I was seeing, is that application
programs push buffers to DWM. Yet, there appears to be no
temporal aspect. If a frame is 20 milliseconds late, and sitting
in a queue, that serves no purpose. The queue needs to discard
old information.

Really, double buffering should be enough, and the
two buffers need never be discarded, until the program
closes and then both would be harvested by the OS. By using
double buffering, the buffers avoid staleness as an issue.

What I think I was seeing, is buffers sitting in limbo. They
were charged to Firefox (Firefox has the elevated memory usage,
not DWM.exe).

I would not know how to account, for DWM not being able
to garbage collect the memory it manages (as composited
windows). While the windows could be pushed to the video card
one at a time, presumably DWM knows when that info is stale,
and an incoming item is newer than the other buffer.

The whole thing just smells of "bad architecture". For
so many bugs of this sort, to have shown up in the past.

Compositing is done via "Z axis priority". When you bring
a window to the front, you assign a Z value which is higher
than any other window. The video card has some dedicated
hardware, for sorting items on the Z axis and determining
which part(s) of a window are visible. This saves the CPU from
having to compute "expose events", which is the method
before compositing came along. Expose events are intercepted
by programs, and the event would say "please redraw this
here 200x400 patch of your screen". And that's how the
patchwork screen image was maintained. AS far as I was
concerned, expose events worked just fine, and we did not
need compositing. But since Apple had it... everyone
else needed it. Linux included (Compiz).

Paul