EDIT4 We now have an answer. Thank you for everyone participating in the test, esp Jamie. Since the answer was deleted, here's the short summary: Win10 introduces memory compression, making this kind of testing difficult and partially pointless. If (on Win8 x64) you try to disable pagefile and write a test app to allocate memory, you'll likely run into allocation fail long before the core is exhausted (Out of CC). What Jamie did was write an app to perform millions of small allocations that did in fact succeed of using every last scrap of ram with no low memory warning. So the mechanism simply does not exist on Win 8 anymore, if you disable pagefile, the first warning you will get is a crash.
WRT failing "normal size" memory allocation while having plenty of CC left is probably due to fragmentation.
With 8GB or 6GB windows 8.1 x64 machine you get a low memory warning if your free RAM drops below about 20% of the total system RAM amount (1.6GB and 1.2GB, respectively) AND there is no more space in the pagefile. If pagefile space is available, physical memory will be allocated into pagefile to keep the 20% or RAM in reserve. So if you're playing Skyrim with a lot of mods and get a low memory warning, you'll probably see the pagefile being completely full and a bit under 20% of RAM being available.
Has anyone tried what is the limit with a 16GB windows machine? Does it extend with no limit i.e. you would receive a low memory warning at 3.2GB?
The easiest way to try this is to disable pagefile altogether or set it to a low value (like 1GB) and then start several apps with high memory use and/or just use this little utility: http://www.soft.tahionic.com/download-memalloc/
I'd test this myself but I have no access to a PC with 16GB (or more!) ram.
In Win8.1 the actual memory use figure is a bit harder to see as performance monitor does not show you pagefile usage. But task manager gives you the "committed" value that shows total memory in use (including pagefile)
Edit: Process explorer system information is probably the best to monitor how memory is being used. Commit charge and limit is the relevant bit here, if you have no pagefile, Commit limit = RAM and you should get a low memory warning when you get ~81% commit charge.
Edit2: To make it even more unambiguous, here's a pseudocode for the two cases I'm asking about
Case A no limit how large the minimum free memory (available commit charge) can grow before warning is issued:
if (CC/CL) > 0.8 then print "low memory warning"
Case B The minimum free memory (available commit charge) is limited to some absolute value and no warning is issued before it is crossed:
if (CC/CL) > 0.8 and if (CL-CC) < 2048MB then print "low memory warning"
Edit3: It turns out Windows 10 compresses memory when it runs low enough on actual RAM. This naturally makes this test more difficult to perform. You'd can still exhaust the available ram to be sure but windows will compress malloc with zero values quite efficiently. In Win8.1 x64 and earlier it's a simple task.
I'm currently having the misfortune to have to use 4GB Windows 7 x64 box. In this system Windows tries to keep ~800MB of physical memory available. This is of course the familiar 20% slice. And it hurts a lot worse than the 1.6GB "reserve" on 8GB box.
I see moderator deleted my answer where I summarized Jamie's findings using a bespoke program written to exhaust the core. Thanks for that.