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From "Sylvain Lebresne (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-3862) RowCache misses Updates
Date Mon, 06 Feb 2012 18:04:12 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-3862?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13201436#comment-13201436
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Sylvain Lebresne commented on CASSANDRA-3862:
---------------------------------------------

I believe you are absolutely right that this is a bug.

Unfortunately I don't think including the memtables during cache reads really solves it. If
you miss an update, it won't ever get added to the cached row, but the update itself will
be flushed at some point and thus not be in any memtable anymore.

One partial solution I see could be that when a read 'reads for caching', it starts adding
some sentinel object in the cache for the given row key. That sentinel would need to be an
actual (empty) row but marked with the fact it's only a sentinel. When a write look if the
row is cache, if it's a sentinel we would add the write to the sentinel. Once the read returns
and we actually put the row in cache, we would it (atomically) with the content of the sentinel.
A read that check the cache and see a sentinel would just skip the cache (and would not put
it's result into the cache). Adapting that to the serializingCache is trivial.

Unfortunately, this is not perfect because this would screw counters. Though I guess for counters
we could do the same thing as we would do for the serializingCache, i.e, if a read that 'reads
for caching' see that the sentinel is not empty, we would just not cache the result (i.e,
a row would be cache only if we are sure no write were done concurrently to the read).
                
> RowCache misses Updates
> -----------------------
>
>                 Key: CASSANDRA-3862
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-3862
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 1.0.7
>            Reporter: Daniel Doubleday
>         Attachments: 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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