RE: [PATCHv6 0/8] zswap: compressed swap caching

[Dan Magenheimer]

> From: Seth Jennings [mailto:sjenning@xxxxxxxxxxxxxxxxxx]
> Subject: Re: [PATCHv6 0/8] zswap: compressed swap caching
> 
> On 02/21/2013 09:50 AM, Dan Magenheimer wrote:
> >> From: Seth Jennings [mailto:sjenning@xxxxxxxxxxxxxxxxxx]
> >> 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
> 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.

For slab, I agree that makes good sense.  But slub (the default allocator)
falls back, I think, to order-0 if order-2 fails.
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