[PATCHv6 0/8] zswap: compressed swap caching

[Ric Mason]

Hi Seth,
On 02/22/2013 02:25 AM, Seth Jennings wrote:
> On 02/21/2013 09:50 AM, Dan Magenheimer wrote:
>>> From: Seth Jennings [mailto:sjenning at linux.vnet.ibm.com]
>>> Subject: [PATCHv6 0/8] zswap: compressed swap caching
>>>
>>> Changelog:
>>>
>>> v6:
>>> * fix improper freeing of rbtree (Cody)
>> Cody's bug fix reminded me of a rather fundamental question:
>>
>> Why does zswap use a rbtree instead of a radix tree?
>>
>> Intuitively, I'd expect that pgoff_t values would
>> have a relatively high level of locality AND at any one time
>> the set of stored pgoff_t values would be relatively non-sparse.
>> This would argue that a radix tree would result in fewer nodes
>> touched on average for lookup/insert/remove.
> I considered using a radix tree, but I don't think there is a compelling
> reason to choose a radix tree over a red-black tree in this case
> (explanation below).
>
>  From a runtime standpoint, a radix tree might be faster.  The swap
> offsets will be largely in linearly bunched groups over the indexed
> range.  However, there are also memory constraints to consider in this
> particular situation.
>
> Using a radix tree could result in intermediate radix_tree_node
> allocations in the store (insert) path in addition to the zswap_entry
> allocation.  Since we are under memory pressure, using the red-black

Then in which case radix tree is prefer and in which case redblack tree 
is better?

> tree, whose metadata is included in the struct zswap_entry, reduces the
> number of opportunities to fail.
>
> On my system, the radix_tree_node structure is 568 bytes.  The
> radix_tree_node cache requires 4 pages per slab, an order-2 page
> allocation.  Growing that cache will be difficult under the pressure.
>
> In my mind, cost of even a single node allocation failure resulting in
> an additional page swapped to disk will more that wipe out any possible
> performance advantage using a radix tree might have.
>
> Thanks,
> Seth
>
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