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Escaping the ARC patent

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: Escaping the ARC patent
Date: 2005-02-03 22:27:29
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Lists: pgsql-hackers
I've been doing a bit of research on $subj, and coming to the conclusion
that the ARC patent is a lot narrower than it might appear.  In fact
most of the parts of the algorithm that we actually want have prior art.
I looked in particular at Johnson and Shasha's well-known "2Q" paper,
published in 1994 (  This paper
describes the use of two lists, which they call A1 and Am (as opposed to
ARC's T1 and T2) but the basic principle is the same: a page goes into
A1 on first use, and doesn't get to Am unless used a second time before
aging out of the cache.  2Q also includes a list of pages that have
recently been in the cache but no longer are.  So the actually
patentable parts of ARC are just some rather narrow decisions about the
management of these lists, in particular the use of a target T1len to
dynamically adapt the sizes of the lists.  The 2Q paper proposes using
fixed fractions of the total available space for each list --- and it
includes statistics showing that the algorithm isn't excessively
sensitive to the exact values used, so ARC's claimed "self tuning"
advantage isn't all that great after all.

These conclusions are borne out by a close reading of the patent
application (which is at
if you want to look for yourself).  Claim 1 reads

1. A method for adaptively managing pages in a cache memory with a
variable workload, comprising: maintaining the cache memory into a first
list L1 and a second list L2; wherein the cache memory has a capacity to
store c pages; and adaptively distributing the workload between the
first list L1 and the second list L2, to a total capacity of c pages.

Given the prior art, the critical word in this sentence is "adaptively";
take that out and you have nothing that wasn't published long before.
If we remove the adaptivity --- ie, just use a fixed division of list
sizes --- we escape claim 1 and all the other claims that depend on it.

The only other claim that isn't dependent on claim 1 or a restatement of
it is

45. A method for adaptively managing pages in a memory, comprising:
defining a cache memory; defining a cache directory; organizing the
cache directory into fours disjoint lists of pages: list T1, list T2,
list B1, and list B2; and wherein the cache memory contains pages that
are members of any of the list T1 or the list T2.

So if we use non-variable sizes of T1/T2 and don't use the four-way
list structure to manage remembrance of pages-formerly-in-cache,
we escape the patent.  But we still have scan resistance, which is the
main thing that ARC was going to buy us.  Pages that are scanned only
once don't get out of A1 and so aren't able to swamp out pages
referenced multiple times.

After reading the 2Q paper my inclination is to use exactly Johnson and
Shasha's "simplified 2Q" algorithm, which uses just A1 and Am with no
remembrance of formerly cached pages.  Their "full 2Q" algorithm strikes
me as a tad bizarre because it will only promote a page into Am after it
has fallen out of A1, ie, it takes two physical reads of the page to
get into Am.  That's just weird.  I think that pages should advance from
A1 into Am on second reference.  Given that, you don't need any
remembrance of pages that were formerly in A1, which basically halves
the memory overhead of the ARC algorithm.

An advantage of heading in this direction (as opposed to, say, LRU/k
or other algorithms) is that this represents a direct simplification
of the ARC code we have now.  We can probably implement it almost
entirely by deletions from freelist.c, with little newly written code.
That gives me a whole lot more confidence that the result will be
reliable enough to back-patch into 8.0.*.


			regards, tom lane


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Subject: Re: Escaping the ARC patent
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