On Fri, 26 Jan 2007, Christoph Lameter wrote:
On Thu, 25 Jan 2007, Mel Gorman wrote:
The following 8 patches against 2.6.20-rc4-mm1 create a zone called
ZONE_MOVABLE that is only usable by allocations that specify both __GFP_HIGHMEM
and __GFP_MOVABLE. This has the effect of keeping all non-movable pages
within a single memory partition while allowing movable allocations to be
satisified from either partition.
For arches that do not have HIGHMEM other zones would be okay too it
seems.
It would, but it'd obscure the code to take advantage of that.
The size of the zone is determined by a kernelcore= parameter specified at
boot-time. This specifies how much memory is usable by non-movable allocations
and the remainder is used for ZONE_MOVABLE. Any range of pages within
ZONE_MOVABLE can be released by migrating the pages or by reclaiming.
The user has to manually fiddle around with the size of the unmovable
partition until it works?
They have to fiddle with the size of the unmovable partition if their
workload uses more unmovable kernel allocations than expected. This was
always going to be the restriction with using zones for partitioning
memory. Resizing zones on the fly is not really an option because the
resizing would only work reliably in one direction.
The anti-fragmentation code could potentially be used to have subzone
groups that kept movable and unmovable allocations as far apart as
possible and at opposite ends of a zone. That approach has been kicked a
few times because of complexity.
When selecting a zone to take pages from for ZONE_MOVABLE, there are two
things to consider. First, only memory from the highest populated zone is
used for ZONE_MOVABLE. On the x86, this is probably going to be ZONE_HIGHMEM
but it would be ZONE_DMA on ppc64 or possibly ZONE_DMA32 on x86_64. Second,
the amount of memory usable by the kernel will be spreadly evenly throughout
NUMA nodes where possible. If the nodes are not of equal size, the amount
of memory usable by the kernel on some nodes may be greater than others.
So how is the amount of movable memory on a node calculated?
Subtle difference. The amount of unmovable memory is calculated per node.
Evenly
distributed?
As evenly as possible.
There are some NUMA architectures that are not that
symmetric.
I know, it's why find_zone_movable_pfns_for_nodes() is as complex as it
is. The mechanism spreads the unmovable memory evenly throughout all
nodes. In the event some nodes are too small to hold their share, the
remaining unmovable memory is divided between the nodes that are larger.
By default, the zone is not as useful for hugetlb allocations because they
are pinned and non-migratable (currently at least). A sysctl is provided that
allows huge pages to be allocated from that zone. This means that the huge
page pool can be resized to the size of ZONE_MOVABLE during the lifetime of
the system assuming that pages are not mlocked. Despite huge pages being
non-movable, we do not introduce additional external fragmentation of note
as huge pages are always the largest contiguous block we care about.
The user already has to specify the partitioning of the system at bootup
and could take the huge page sizes into account.
Not in all cases. Some systems will not know how many huge pages they need
in advance because it is used as a batch system running jobs as requested.
The zone allows an amount of memory to be set aside that can be
*optionally* used for hugepages if desired or base pages if not. Between
jobs, the hugepage pool can be resized up to the size of ZONE_MOVABLE.
The other case is ever supporting memory hot-remove. Any memory within
ZONE_MOVABLE can potentially be removed by migrating pages and off-lined.
Also huge pages may have variable sizes that can be specified on bootup
for IA64. The assumption that a huge page is always the largest
contiguous block is *not true*.
I didn't say they were the largest supported contiguous block, I said they
were the largest contiguous block we *care* about. Right now, it is
assumed that variable pages are not supported at runtime. If they were,
some smarts would be needed to keep huge pages of the same size together
to control external fragmentation but that's about it.
The huge page sizes on i386 and x86_64 platforms are contigent on
their page table structure. This can be completely different on other
platforms.
The size doesn't really make much difference to the mechanism.
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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