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From "Sylvain Lebresne (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-3862) RowCache misses Updates
Date Tue, 14 Feb 2012 11:17:00 GMT

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Sylvain Lebresne commented on CASSANDRA-3862:
---------------------------------------------

The cleanup lgtm.

For the change to invalidateCacheRow however, I wonder if it's worth it. By waiting when we
found a sentinel, we may have writes waiting on a read to complete, which could involve a
non negligible latency spike. On the other side, if we just leave the sentinel in that case,
the only risk we take is that a read may end up putting tombstone in the cache that are already
expired. But it doesn't seem like a big deal, especially given that it will very rarely happen.

But in any case, you're right about negating the loop condition. 
                
> RowCache misses Updates
> -----------------------
>
>                 Key: CASSANDRA-3862
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-3862
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 0.6
>            Reporter: Daniel Doubleday
>            Assignee: Sylvain Lebresne
>             Fix For: 1.1.0
>
>         Attachments: 3862-cleanup.txt, 3862-v2.patch, 3862.patch, 3862_v3.patch, include_memtables_in_rowcache_read.patch
>
>
> While performing stress tests to find any race problems for CASSANDRA-2864 I guess I
(re-)found one for the standard on-heap row cache.
> During my stress test I hava lots of threads running with some of them only reading other
writing and re-reading the value.
> This seems to happen:
> - Reader tries to read row A for the first time doing a getTopLevelColumns
> - Row A which is not in the cache yet is updated by Writer. The row is not eagerly read
during write (because we want fast writes) so the writer cannot perform a cache update
> - Reader puts the row in the cache which is now missing the update
> I already asked this some time ago on the mailing list but unfortunately didn't dig after
I got no answer since I assumed that I just missed something. In a way I still do but haven't
found any locking mechanism that makes sure that this should not happen.
> The problem can be reproduced with every run of my stress test. When I restart the server
the expected column is there. It's just missing from the cache.
> To test I have created a patch that merges memtables with the row cache. With the patch
the problem is gone.
> I can also reproduce in 0.8. Haven't checked 1.1 but I haven't found any relevant change
their either so I assume the same aplies there.

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