Re: [Qemu-devel] [PATCH 03/15] coroutine-ucontext: reduce stack size to 64kB

[Peter Lieven]

Am 28.06.2016 um 12:54 schrieb Paolo Bonzini:
> On 28/06/2016 11:01, Peter Lieven wrote:
> > evaluation with the recently introduced maximum stack size monitoring revealed
> > that the actual used stack size was never above 4kB so allocating 1MB stack
> > for each coroutine is a lot of wasted memory. So reduce the stack size to
> > 64kB which should still give enough head room.
> If we make the stack this much smaller, there is a non-zero chance of
> smashing it.  You must add a guard page if you do this (actually more
> than one because QEMU will happily have stack frames as big as 16 KB).
> The stack counts for RSS but it's not actually allocated memory, so why
> does it matter?

Is there an easy way to determinate how much of the RSS is actually allocated?
I erroneously it was all allocated....

So as for the stack, the MAP_GROWSDOWN is it really important? Will the kernel
allocate all pages of the stack otherwise if the last page is written?

I am asking because I don't know if MAP_GROWSDOWN is a good idea as Peter
mentioned there were discussions to deprecate it.

Peter