Re: physical memory chunk statistics?

On Sat, Dec 21, 2024 at 05:03:45PM +0000, Bjoern A. Zeeb wrote:
> Hi,
> 
> upon boot we display our physical memory junks nicely.
> 
> Physical memory chunk(s):
> 0x0000000000001000 - 0x000000000009ffff, 651264 bytes (159 pages)
> 0x0000000000101000 - 0x0000000012df5fff, 315576320 bytes (77045 pages)
> 0x0000000013c00000 - 0x0000000013d87fff, 1605632 bytes (392 pages)
> 0x0000000016401000 - 0x000000001e920fff, 139591680 bytes (34080 pages)
> 0x000000001eb1b000 - 0x000000001f73afff, 12713984 bytes (3104 pages)
> 
> 
> Do we have any way on a running system to export some statistics of
> how much each of them is used up?  Something like [Use, Requests]?
Look at vm.phys_segs.  These are segments of physical memory as seen
by the phys allocator (vm_phys.c).  It is mostly the same chunks as were
reported e.g. by UEFI parser at boot, but after the kernel initial memory
and other stuff took some pages before vm was initialized.

> 
> 
> Say one wanted to debug the good old lower 4GB contigmalloc failing
> problem (and example but also something I am just facing again).
> How would one do that?  The immediate questions are:
> (a) how much of the available lower physical 4G chunks are used?
> (b) how much fragmentation is there roughly?
> (c) what's the largest contig. chunk avail?
> 
Then look at vm.phys_free.  First, there are free lists.  On amd64, freelist
0 is the normal freelist (all pages except freelist 1), and freelist 1 is
the pages below 4G (ie. DMA32).  See sys/amd64/include/vmparam.h for
definitions and comments.

The sysctl reports the number of free pages clustered by the contigous
order.  Pools are mostly internal to vm, they allow to distinguish allocs
from direct map (e.g. for UMA page allocs) vs. generic contig allocs.

People actively working with allocators could correct me.

> Given (c) is likely harder and expensive (a) and (b) could at least
> give an idea.  Did we ever solve that?

vm.phys_free should give the answer up to the supported order size.

That said, a radical solution for the problem of memory below 4G is
to use IOMMU.  It might be enabled just for specific PCI device.