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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8894) Our default buffer size for (uncompressed) buffered reads should be smaller, and based on the expected record size
Date Thu, 25 Jun 2015 10:32:05 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8894?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14600972#comment-14600972
] 

Benedict commented on CASSANDRA-8894:
-------------------------------------

The OS page cache is also page aligned. The idea is to ensure we only touch whole pages in
the page cache, so we need to ensure we only request data at page-aligned positions in the
file. For the first read, we could optionally truncate the amount we read by starting at the
position we are reading from (but stop at the same page-aligned boundary that it would if
it read those bytes), but this is unlikely to make a material difference.

> 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: Stefania
>             Fix For: 3.x
>
>
> 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.



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