Benedict created CASSANDRA-8894:
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Summary: Our default buffer size for (uncompressed) buffered reads
should be smaller, and based on the expected record size
Key: CASSANDRA-8894
URL: https://issues.apache.org/jira/browse/CASSANDRA-8894
Project: Cassandra
Issue Type: Improvement
Components: Core
Reporter: Benedict
Assignee: Benedict
Fix For: 3.0
A large contributor to slower buffered reads than mmapped is likely that we
read a full 64Kb at once, when average record sizes may be as low as 140 bytes
on our stress tests. The TLB has only 128 entries on a modern core, and each
read will touch 16 of these, meaning we are unlikely to almost ever be hitting
the TLB, and will be incurring at least 15 unnecessary misses each time (as
well as the other costs of larger than necessary accesses). When working with
an SSD there is little to no benefit reading more than 4Kb at once, and in
either case reading more data than we need is wasteful. So, I propose selecting
a buffer size that is the next larger power of 2 than our average record size
(with a minimum of 4Kb), so that we expect to read in one operation. I also
propose that we create a pool of these buffers up-front, and that we ensure
they are all exactly aligned to a virtual page, so that the source and target
operations each touch exactly one virtual page per 4Kb of expected record size.
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