On 04/10/2013 01:18:54 PM, Seth Jennings wrote:
This patch adds a documentation file for zsmalloc at
Documentation/vm/zsmalloc.txt
Docs acked-by: Rob Landley <rob@xxxxxxxxxxx>
Literary criticism below:
Signed-off-by: Seth Jennings <sjenning@xxxxxxxxxxxxxxxxxx>
---
Documentation/vm/zsmalloc.txt | 68
+++++++++++++++++++++++++++++++++++++++++++
1 file changed, 68 insertions(+)
create mode 100644 Documentation/vm/zsmalloc.txt
diff --git a/Documentation/vm/zsmalloc.txt
b/Documentation/vm/zsmalloc.txt
new file mode 100644
index 0000000..85aa617
--- /dev/null
+++ b/Documentation/vm/zsmalloc.txt
@@ -0,0 +1,68 @@
+zsmalloc Memory Allocator
+
+Overview
+
+zmalloc a new slab-based memory allocator,
+zsmalloc, for storing compressed pages.
zmalloc a new slab-based memory allocator, zsmalloc? (How does one
zmalloc zsmalloc?)
Out of curiosity, what does zsmalloc stand for, anyway?
It is designed for
+low fragmentation and high allocation success rate on
+large object, but <= PAGE_SIZE allocations.
1) objects
2) maybe "large objects for <= PAGE_SIZE"...
+zsmalloc differs from the kernel slab allocator in two primary
+ways to achieve these design goals.
+
+zsmalloc never requires high order page allocations to back
+slabs, or "size classes" in zsmalloc terms. Instead it allows
+multiple single-order pages to be stitched together into a
+"zspage" which backs the slab. This allows for higher allocation
+success rate under memory pressure.
+
+Also, zsmalloc allows objects to span page boundaries within the
+zspage. This allows for lower fragmentation than could be had
+with the kernel slab allocator for objects between PAGE_SIZE/2
+and PAGE_SIZE. With the kernel slab allocator, if a page compresses
+to 60% of it original size, the memory savings gained through
+compression is lost in fragmentation because another object of
I lean towards "are lost", but it's debatable. (Savings are plural, but
savings could also be treated as a mass noun like water/air/bison that
doesn't get pluralized because you can't count instances of a liquid.
No idea which is more common.)
+the same size can't be stored in the leftover space.
+
+This ability to span pages results in zsmalloc allocations not being
+directly addressable by the user. The user is given an
+non-dereferencable handle in response to an allocation request.
+That handle must be mapped, using zs_map_object(), which returns
+a pointer to the mapped region that can be used. The mapping is
+necessary since the object data may reside in two different
+noncontigious pages.
Presumably this allows packing of unmapped entities if you detect
fragmentation and are up for a latency spike?
Rob
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