On Jan 29, 2005, at 7:42 PM, Laurence Harris, Aandi, and Skip wrote:
Now I need a way to be able to check if I have sufficient memory after
my allocations so that the Mac can actually keep running without
crashing in calls such as DrawPicture.
That's basically what I said, though it's not quite that simple. Can
your
application have multiple windows? Can they be any size? Could the
user open
a save file dialog when memory use is high? Point is, it's very hard to
predict how much memory the system will need at various times.
There's another VM issue that can bite you. When you allocate memory,
say
with NewHandle(), the system doesn't really do much more than make sure
there's room within your 4GB space. It does not, as you might suppose,
verify that there's sufficient space on disk for VM to actually
provide all
the memory you requested. For example, suppose you call
NewHandle(.9GB).
Suppose further that this is allowable given the available space in
your
memory space. But suppose there is only .4GB left on the hard drive
for VM
to grow swap files. Uh oh. The Memory Manager will let you allocate the
handle, but if you try to actually write to all of it, your Mac will
fall
down. And Apple can't fully address this problem. To do that, the VM
system
would have to reserve 4BG of disk space for every running application,
and
that's just not feasible. I have ten applications running at the
moment, and
that would mean the system had to reserve 40GB of my 60GB disk just
for VM.
One possible strategy that I am working on is this. I pick a buffer
size, some guestimate as to what the application is going to need to
keep running. I haven't come up with a real value for this yet, but in
my tests, I am just grabbing a 64 MByte chunk.
Just prior to doing any operation that could allocate a huge amount of
memory and fail, I first attempt to grab a 64 MByte chunk of RAM, as a
contingency buffer. Allocate it, do nothing with it, except hold onto
it. I then proceed with my regular memory allocations. IF the
allocations fail, they roll back. IF they succeed, great. In either
case, I then let go of the contingency block, and cross my fingers that
there is enough memory available for the application to keep running
nicely and not choke somewhere in which I have no control. So
essentially all big consumers of memory in my application would be
framed by a grab at a contingency buffer, and a release of it
afterwards. IF you can't get the buffer, you can't proceed with the
requested operation.
Anyone see any problems with such a strategy? And does anyone have any
suggestions as to a good strategy for figuring out the size of this
"set-aside" memory?
Skip
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