Re: [PATCHv5 2/8] zsmalloc: add documentation

From: Ric Mason
Date: Fri Mar 01 2013 - 01:47:28 EST

Next message: Mark Brown: "Re: [PATCH 1/5] regmap: irq: call pm_runtime_put inpm_runtime_get_sync failed case"
Previous message: Marc MERLIN: "Re: Reproduceable SATA lockup on 3.7.8 with SSD"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

On 02/25/2013 11:18 PM, Seth Jennings wrote:

On 02/23/2013 06:37 PM, Ric Mason wrote:
On 02/23/2013 05:02 AM, Seth Jennings wrote:
On 02/21/2013 08:56 PM, Ric Mason wrote:
On 02/21/2013 11:50 PM, Seth Jennings wrote:
On 02/21/2013 02:49 AM, Ric Mason wrote:
On 02/19/2013 03:16 AM, Seth Jennings wrote:
On 02/16/2013 12:21 AM, Ric Mason wrote:
On 02/14/2013 02:38 AM, Seth Jennings wrote:
This patch adds a documentation file for zsmalloc at
Documentation/vm/zsmalloc.txt

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. It is designed for
+low fragmentation and high allocation success rate on
+large object, but <= PAGE_SIZE allocations.
+
+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
+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.

Do you mean the reason of to use a zsmalloc object must map after
malloc is object data maybe reside in two different nocontiguous
pages?

Yes, that is one reason for the mapping. The other reason (more
of an
added bonus) is below.

+
+For 32-bit systems, zsmalloc has the added benefit of being
+able to back slabs with HIGHMEM pages, something not possible

What's the meaning of "back slabs with HIGHMEM pages"?
By HIGHMEM, I'm referring to the HIGHMEM memory zone on 32-bit
systems
with larger that 1GB (actually a little less) of RAM. The upper
3GB
of the 4GB address space, depending on kernel build options, is not
directly addressable by the kernel, but can be mapped into the
kernel
address space with functions like kmap() or kmap_atomic().

These pages can't be used by slab/slub because they are not
continuously mapped into the kernel address space. However, since
zsmalloc requires a mapping anyway to handle objects that span
non-contiguous page boundaries, we do the kernel mapping as part of
the process.

So zspages, the conceptual slab in zsmalloc backed by single-order
pages can include pages from the HIGHMEM zone as well.

Thanks for your clarify,
http://lwn.net/Articles/537422/, your article about zswap in lwn.
"Additionally, the kernel slab allocator does not allow
objects that
are less
than a page in size to span a page boundary. This means that if an
object is
PAGE_SIZE/2 + 1 bytes in size, it effectively use an entire page,
resulting in
~50% waste. Hense there are *no kmalloc() cache size* between
PAGE_SIZE/2 and
PAGE_SIZE."
Are your sure? It seems that kmalloc cache support big size, your
can
check in
include/linux/kmalloc_sizes.h

Yes, kmalloc can allocate large objects > PAGE_SIZE, but there are no
cache sizes _between_ PAGE_SIZE/2 and PAGE_SIZE. For example, on a
system with 4k pages, there are no caches between kmalloc-2048 and
kmalloc-4096.

kmalloc object > PAGE_SIZE/2 or > PAGE_SIZE should also allocate from
slab cache, correct? Then how can alloc object w/o slab cache which?
contains this object size objects?

I have to admit, I didn't understand the question.
object is allocated from slab cache, correct? There two kinds of slab
cache, one is for general purpose, eg. kmalloc slab cache, the other
is for special purpose, eg. mm_struct, task_struct. kmalloc object >
PAGE_SIZE/2 or > PAGE_SIZE should also allocated from slab cache,
correct? then why you said that there are no caches between
kmalloc-2048 and kmalloc-4096?

Ok, now I get it. Yes, I guess I should qualified here that there are
no _kmalloc_ caches between PAGE_SIZE/2 and PAGE_SIZE.

Why I have?

dma-kmalloc-8192 0 0 8192 4 8 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-4096 0 0 4096 8 8 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-2048 0 0 2048 16 8 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-1024 0 0 1024 32 8 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-512 32 32 512 32 4 : tunables 0 0 0 : slabdata 1 1 0
dma-kmalloc-256 0 0 256 32 2 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-128 0 0 128 32 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-64 0 0 64 64 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-32 0 0 32 128 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-16 0 0 16 256 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-8 0 0 8 512 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-192 0 0 192 21 1 : tunables 0 0 0 : slabdata 0 0 0
dma-kmalloc-96 0 0 96 42 1 : tunables 0 0 0 : slabdata 0 0 0
kmalloc-8192 100 100 8192 4 8 : tunables 0 0 0 : slabdata 25 25 0
kmalloc-4096 178 216 4096 8 8 : tunables 0 0 0 : slabdata 27 27 0
kmalloc-2048 229 304 2048 16 8 : tunables 0 0 0 : slabdata 19 19 0
kmalloc-1024 832 832 1024 32 8 : tunables 0 0 0 : slabdata 26 26 0
kmalloc-512 2016 2016 512 32 4 : tunables 0 0 0 : slabdata 63 63 0
kmalloc-256 2203 2368 256 32 2 : tunables 0 0 0 : slabdata 74 74 0
kmalloc-128 2026 2464 128 32 1 : tunables 0 0 0 : slabdata 77 77 0
kmalloc-64 27584 27584 64 64 1 : tunables 0 0 0 : slabdata 431 431 0
kmalloc-32 19334 20864 32 128 1 : tunables 0 0 0 : slabdata 163 163 0
kmalloc-16 6912 6912 16 256 1 : tunables 0 0 0 : slabdata 27 27 0
kmalloc-8 17408 17408 8 512 1 : tunables 0 0 0 : slabdata 34 34 0
kmalloc-192 8006 8946 192 21 1 : tunables 0 0 0 : slabdata 426 426 0
kmalloc-96 19828 19992 96 42 1 : tunables 0 0 0 : slabdata 476 476 0
kmem_cache_node 384 384 32 128 1 : tunables 0 0 0 : slabdata 3 3 0
kmem_cache 160 160 128 32 1 : tunables 0 0 0 : slabdata 5 5 0

Yes, one can create caches of a particular size. However that doesn't
work well for zswap because the compressed pages vary widely and size
and, imo, it doesn't make sense to create a bunch of caches very
granular in size.

Plus having granular caches doesn't solve the fragmentation issue
caused by the storage of large objects.

Thanks,
Seth

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxxx For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

Next message: Mark Brown: "Re: [PATCH 1/5] regmap: irq: call pm_runtime_put inpm_runtime_get_sync failed case"
Previous message: Marc MERLIN: "Re: Reproduceable SATA lockup on 3.7.8 with SSD"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]