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

[Seth Jennings]

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.

Thanks,
Seth

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