There are alternatives to LRU, which is generally chosen for being extremely
simple to implement, fast, and has a reasonable hit rate. The
Greedy-Dual-Size-Frequency policy may be more appropriate for memcached as it
accounts a value's weight. I doubt that there's a lot of value of changing the
current design, but there are alternatives to approaches that would need to be
considered if GC was a serious consideration.
________________________________
From: Jakub Łopuszański <jakub.lopuszan...@nasza-klasa.pl>
To: memcached@googlegroups.com
Sent: Fri, July 23, 2010 12:16:16 AM
Subject: Re: Using PCIe SSDs instead of RAM
On Fri, Jul 23, 2010 at 8:47 AM, dormando <dorma...@rydia.net> wrote:
I tried.
>
>Try the engine branch?
>
>
>
I guess, I'll have to at some point.
Just wanted to say, that LRU was designed as an algorithm for a uniform cost
model, where all elements are almost equally important (have the same cost of
miss) and the only thing that distinguishes them is the pattern of accesses.
This is clearly not a good model for memcache, where: some elements are totally
unimportant as they have already expired, some elements are larger than the
others, some are always processed in batches (multigets), and so on. In my
opinion GC moves the reality closer to the model, by removing unimportant
elements, so if you want LRU to work correctly you should at least perform GC.
You could also try to modify LRU to model that one large item actually occupies
space that could be better utilies by several small elements (this is also a
simple change). If you fill comfortable without GC, I am OK with that, just do
not suggest, that GC is against LRU.
