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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-8894) Our default buffer size for (uncompressed) buffered reads should be smaller, and based on the expected record size
Date Fri, 17 Jul 2015 09:49:04 GMT


Benedict commented on CASSANDRA-8894:

A few comments on the stress testing:

* The blob_id population doesn't need to be constrained (it defaults to something like 1..100B)
* To perform the inserts, we want to ensure we construct a dataset large enough to spill to
disk, i.e. we want to probably insert at least 100M items (perhaps 200M+) if they're only
~50 bytes each.
* We probably want to run with slightly more threads, say 300

The graphs don't appear to actually be broken that were produced: the stress run was simply
extremely brief, since it only operated over 100K items :)

At risk of sounding like a broken record to everyone, it can help to use K, M, B syntax for
your numbers in the profile/command line.

> Our default buffer size for (uncompressed) buffered reads should be smaller, and based
on the expected record size
> ------------------------------------------------------------------------------------------------------------------
>                 Key: CASSANDRA-8894
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>            Assignee: Stefania
>              Labels: benedict-to-commit
>             Fix For: 3.x
>         Attachments: 8894_25pct.yaml, 8894_5pct.yaml, 8894_tiny.yaml
> 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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