mmap performance and memory use

Alan Cox alc at
Fri Oct 28 05:38:07 UTC 2011

On 10/26/2011 06:23, Svatopluk Kraus wrote:
> Hi,
> well, I'm working on new port (arm11 mpcore) and pmap_enter_object()
> is what I'm debugging rigth now. And I did not find any way in
> userland how to force kernel to call pmap_enter_object() which makes
> SUPERPAGE mapping without promotion. I tried to call mmap() with
> MAP_PREFAULT_READ without success. I tried to call madvise() with
> MADV_WILLNEED without success too.

mmap() should call pmap_enter_object() if MAP_PREFAULT_READ was 
specified.  I'm surprised to hear that it's not happening for you.

> To make SUPERPAGE mapping, it's obvious that all physical pages under
> SUPERPAGE must be allocated in vm_object. And SUPERPAGE mapping must
> be done before first access to them, otherwise a promotion is on the
> way. MAP_PREFAULT_READ does nothing with it. If madvice() is used,
> vm_object_madvise() is called but only cached pages are allocated in
> advance. Of coarse, an allocation of all physical memory behind
> virtual address space in advance is not preferred in most situations.
> For example, I want to do some computation on 4M memory space (I know
> that each byte will be accessed) and want to utilize SUPERPAGE mapping
> without promotion, so save 4K page table (i386 machine). However,
> malloc() leads to promotion, mmap() with MAP_PREFAULT_READ doesn't do
> nothing so SUPERPAGE mapping is promoted, and madvice() with
> MADV_WILLNEED calls vm_object_madvise() but because the pages are not
> cached (how can be on anonymous memory), it is not work without
> promotion too.
> So, SUPERPAGE mapping without promotions is fine, but it can be done
> only if physical memory being mapped is already allocated. Is it
> really possible to force that in userland?

To force the allocation of the physical memory?  Right now, the only way 
is for your program to touch the pages.

> Moreover, the SUPERPAGE mapping is made readonly firstly. So, even if
> I have SUPERPAGE mapping without promotion, the mapping is demoted
> after first write, and promoted again after all underlying pages are
> accessed by write. There is 4K page table saving no longer.

Yes, that is all true.  It is possible to change things so that the page 
table pages are reclaimed after a time, and not kept around 
indefinitely.  However, this not high on my personal priority list.  
Before that, it is more likely that I will add an option to avoid the 
demotion on write, if we don't have to copy the entire superpage to do so.


More information about the freebsd-hackers mailing list