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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6746) Reads have a slow ramp up in speed
Date Mon, 24 Mar 2014 23:46:51 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6746?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13945879#comment-13945879
] 

Benedict commented on CASSANDRA-6746:
-------------------------------------

I assumed that the idea is to turn off read-ahead (using FADV_RANDOM) and to always manage
it ourselves. Which may well be an ok idea - but will require some other changes. I must admit
I don't quite follow all of what you're saying: since we always buffer (except for mmapped
files) 64K, WILLNEEDing the entire block seems perfectly acceptable since we have to read
it anyway (it doesn't matter where the column occurs, unless it happens to be in a different
chunk).

I must admit I am left a smidgen generally concerned about the WILLNEED flag and its implications
for treatment of the page after the first use. I would feel more comfortable dropping all
uses of WILLNEED from the codebase, without clearer definitions of the semantics, since we
can probably do without them after CASSANDRA-6916. But I don't feel so strongly about this.

bq. is keeping page cache for the old files creates more jitter and slows down warmup of the
newly created SSTable.

CASSANDRA-6916 should solve this problem.

> Reads have a slow ramp up in speed
> ----------------------------------
>
>                 Key: CASSANDRA-6746
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6746
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>            Reporter: Ryan McGuire
>            Assignee: Benedict
>              Labels: performance
>             Fix For: 2.1 beta2
>
>         Attachments: 2.1_vs_2.0_read.png, 6746-buffered-io-tweaks.png, 6746-patched.png,
6746.blockdev_setra.full.png, 6746.blockdev_setra.zoomed.png, 6746.buffered_io_tweaks.logs.tar.gz,
6746.buffered_io_tweaks.write-flush-compact-mixed.png, 6746.buffered_io_tweaks.write-read-flush-compact.png,
6746.txt, buffered-io-tweaks.patch, cassandra-2.0-bdplab-trial-fincore.tar.bz2, cassandra-2.1-bdplab-trial-fincore.tar.bz2
>
>
> On a physical four node cluister I am doing a big write and then a big read. The read
takes a long time to ramp up to respectable speeds.
> !2.1_vs_2.0_read.png!
> [See data here|http://ryanmcguire.info/ds/graph/graph.html?stats=stats.2.1_vs_2.0_vs_1.2.retry1.json&metric=interval_op_rate&operation=stress-read&smoothing=1]



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