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added some analysis on cache effectiveness.

JINMEI Tatuya 12 years ago
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      doc/design/resolver/03-cache-algorithm.txt

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doc/design/resolver/03-cache-algorithm.txt

@@ -20,3 +20,90 @@ which algorithm is best for that. If it is very critical, then a
 custom algorithm designed for DNS caching makes sense. If it is not,
 custom algorithm designed for DNS caching makes sense. If it is not,
 then we can consider using an STL-based data structure.
 then we can consider using an STL-based data structure.
 
 
+Effectiveness of Cache
+----------------------
+
+First, I'll try to answer the introductory questions.
+
+In some simplified model, we can express the amount of running time
+for answering queries directly from the cache in the total running
+time including that used for recursive resolution due to cache miss as
+follows:
+
+A = r*Q2*/(r*Q2+ Q1*(1-r))
+where
+A: amount of time for answering queries from the cache per unit time
+   (such as sec, 0<=A<=1)
+r: cache hit rate (0<=r<=1)
+Q1: max qps of the server with 100% cache hit
+Q2: max qps of the server with 0% cache hit
+
+Q1 can be measured easily for given data set; measuring Q2 is tricky
+in general (it requires many external queries with unreliable
+results), but we can still have some not-so-unrealistic numbers
+through controlled simulation.
+
+As a data point for these values, see a previous experimental results
+of mine:
+https://lists.isc.org/pipermail/bind10-dev/2012-July/003628.html
+
+Looking at the "ideal" server implementation (no protocol overhead)
+with the set up 90% and 85% cache hit rate with 1 recursion on cache
+miss, and with the possible maximum total throughput, we can deduce
+Q1 and Q2, which are: 170591qps and 60138qps respectively.
+
+This means, with 90% cache hit rate (r = 0.9), the server would spend
+76% of its run time for receiving queries and answering responses
+directly from the cache: 0.9*60138/(0.9*60138 + 0.1*170591) = 0.76.
+
+I also ran more realistic experiments: using BIND 9.9.2 and unbound
+1.4.19 in the "forward only" mode with crafted query data and the
+forwarded server to emulate the situation of 100% and 0% cache hit
+rates.  I then measured the max response throughput using a
+queryperf-like tool.  In both cases Q2 is about 28% of Q1 (I'm not
+showing specific numbers to avoid unnecessary discussion about
+specific performance of existing servers; it's out of scope of this
+memo).  Using Q2 = 0.28*Q1, above equation with 90% cache hit rate
+will be: A = 0.9 * 0.28 / (0.9*0.28 + 0.1) = 0.716. So the server will
+spend about 72% of its running time to answer queries directly from
+the cache.
+
+Of course, these experimental results are too simplified.  First, in
+these experiments we assumed only one external query is needed on
+cache miss.  In general it can be more; however, it may not actually
+too optimistic either: in my another research result:
+http://bind10.isc.org/wiki/ResolverPerformanceResearch
+In the more detailed analysis using real query sample and tracing what
+an actual resolver would do, it looked we'd need about 1.44 to 1.63
+external queries per cache miss in average.
+
+Still, of course, the real world cases are not that simple: in reality
+we'd need to deal with timeouts, slower remote servers, unexpected
+intermediate results, etc.  DNSSEC validating resolvers will clearly
+need to do more work.
+
+So, in the real world deployment Q2 should be much smaller than Q1.
+Here are some specific cases of the relationship between Q1 and Q2 for
+given A (assuming r = 0.9):
+
+70%: Q2 = 0.26 * Q1
+60%: Q2 = 0.17 * Q1
+50%: Q2 = 0.11 * Q1
+
+So, even if "recursive resolution is 10 times heavier" than the cache
+only case, we can assume the server spends a half of its run time for
+answering queries directly from the cache at the cache hit rate of
+90%.  I think this is a reasonably safe assumption.
+
+Now, assuming the number of 50% or more, does this suggest we should
+highly optimize the cache?  Opinions may vary on this point, but I
+personally think the answer is yes.  I've written an experimental
+cache only implementation that employs the idea of fully-rendered
+cached data.  On one test machine (2.20GHz AMD64, using a single
+core), queryperf-like benchmark shows it can handle over 180Kqps,
+while BIND 9.9.2 can just handle 41K qps.  The experimental
+implementation skips some necessary features for a production server,
+and cache management itself is always inevitable bottleneck, so the
+production version wouldn't be that fast, but it still suggests it may
+not be very difficult to reach over 100Kqps in production environment
+including recursive resolution overhead.