Dear KVM Developers:
I have some questions about how KVM hypervisor requests and allocate
physical pages to the VM. I am using kernel version 3.2.14.
I run a microbenchmark in the VM, which declares an array with certain
size and then assigns some value to all the elements in the array,
which causes page fault captured by the hypervisor and allocates the
machine physical pages.
Depending on the array size, I got two different trace results.
(1) When array size == 10MB or 20 MB, the trace file has the page
allocation events for the qemu-kvm process as follows
qemu-kvm-14402 [004] 1912063.581683: kmalloc_node:
call_site=ffffffff813df54a ptr=ffff880779a42800 bytes_req=576
bytes_alloc=1024 gfp_flags=GFP_KERNEL|GFP_REPEAT node=-1
qemu-kvm-14402 [004] 1912063.581701: kmem_cache_alloc_node:
call_site=ffffffff813e302c ptr=ffff8800229ff500 bytes_req=256
bytes_alloc=256 gfp_flags=GFP_KERNEL|GFP_REPEAT node=-1
qemu-kvm-14402 [004] 1912063.581701: kmalloc_node:
call_site=ffffffff813df54a ptr=ffff880779a42800 bytes_req=576
bytes_alloc=1024 gfp_flags=GFP_KERNEL|GFP_REPEAT node=-1
qemu-kvm-14402 [004] 1912063.581710: kmem_cache_alloc_node:
call_site=ffffffff813e302c ptr=ffff8800229ff800 bytes_req=256
bytes_alloc=256 gfp_flags=GFP_KERNEL|GFP_REPEAT node=-1
qemu-kvm-14402 [004] 1912063.581710: kmalloc_node:
call_site=ffffffff813df54a ptr=ffff880779a47800 bytes_req=640
bytes_alloc=1024 gfp_flags=GFP_KERNEL|GFP_REPEAT node=-1
qemu-kvm-14402 [004] 1912063.581728: kmem_cache_alloc_node:
call_site=ffffffff813e302c ptr=ffff8800229ffe00 bytes_req=256
bytes_alloc=256 gfp_flags=GFP_KERNEL|GFP_REPEAT
... ...
(2) When array size == 40MB, the trace file has the page allocation events as
qemu-kvm-14450 [005] 1911006.440538: mm_page_alloc:
page=ffffea0002570f40 pfn=613437 order=0 migratetype=2
gfp_flags=GFP_HIGHUSER_MOVABLE|GFP_ZERO
qemu-kvm-14450 [005] 1911006.440542: mm_page_alloc:
page=ffffea0002577480 pfn=613842 order=0 migratetype=2
gfp_flags=GFP_HIGHUSER_MOVABLE|GFP_ZERO
qemu-kvm-14450 [005] 1911006.440545: mm_page_alloc:
page=ffffea000070a3c0 pfn=115343 order=0 migratetype=2
gfp_flags=GFP_HIGHUSER_MOVABLE|GFP_ZERO
qemu-kvm-14450 [005] 1911006.440549: mm_page_alloc:
page=ffffea0001a9a500 pfn=435860 order=0 migratetype=2
gfp_flags=GFP_HIGHUSER_MOVABLE|GFP_ZERO
qemu-kvm-14450 [005] 1911006.440552: mm_page_alloc:
page=ffffea00016f7e80 pfn=376314 order=0 migratetype=2
gfp_flags=GFP_HIGHUSER_MOVABLE|GFP_ZERO
qemu-kvm-14450 [005] 1911006.440556: mm_page_alloc:
page=ffffea0001a9d700 pfn=436060 order=0 migratetype=2
gfp_flags=GFP_HIGHUSER_MOVABLE|GFP_ZERO
qemu-kvm-14450 [005] 1911006.440559: mm_page_alloc:
page=ffffea0002576880 pfn=613794 order=0 migratetype=2
gfp_flags=GFP_HIGHUSER_MOVABLE|GFP_ZERO
qemu-kvm-14450 [005] 1911006.440563: mm_page_alloc:
page=ffffea00016f7b40 pfn=376301 order=0 migratetype=2
gfp_flags=GFP_HIGHUSER_MOVABLE|GFP_ZERO
qemu-kvm-14450 [005] 1911006.440569: mm_page_alloc:
page=ffffea00016f7f80 pfn=376318 order=0 migratetype=2
gfp_flags=GFP_HIGHUSER_MOVABLE|GFP_ZER
....
----------------------------------------------------------------------------------------------
When size = 10MB and 20MB, it looks like that KVM use
kmem_cache_alloc_node and kmalloc_node to allocate physical pages.
However, when size = 40MB, KVM hypervisor uses mm_page_alloc to
allocator physical pages. The former is based on the slab allocator,
while the latter is directly from the buddy allocator.
So what is the heuristic used by the KVM to determine when to use the
slab allocator or directly from the buddy allocator? Or is there
anything wrong with my trace file? Thanks in advance.
- Hui
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