[
https://issues.apache.org/jira/browse/CASSANDRA-8894?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Benedict updated CASSANDRA-8894:
--------------------------------
Description: 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 32 of these, meaning we are unlikely to almost
ever be hitting the TLB, and will be incurring at least 30 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. (was: 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.)
> 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 32 of these, meaning we are unlikely to almost ever be
> hitting the TLB, and will be incurring at least 30 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.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
