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From "Jonathan Ellis (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-2843) better performance on long row read
Date Mon, 08 Aug 2011 22:02:27 GMT


Jonathan Ellis commented on CASSANDRA-2843:

Some things I'm not thrilled with:

- the IColumnMap name when it does not implement Map interface, and some things it has in
common with Map (iteration) it changes semantics of (iterating values instead of keys).  not
sure what to use instead though, since we already have an IColumnContainer.  Maybe ISortedColumns?
- TSCM and ALCM extending instead of wrapping CSLM/AL, respectively
- unrelated reformatting

Otherwise, lgtm

> better performance on long row read
> -----------------------------------
>                 Key: CASSANDRA-2843
>                 URL:
>             Project: Cassandra
>          Issue Type: New Feature
>            Reporter: Yang Yang
>             Fix For: 1.0
>         Attachments: 2843.patch, 2843_d.patch, 2843_g.patch, fix.diff, microBenchmark.patch,
patch_timing, std_timing
> currently if a row contains > 1000 columns, the run time becomes considerably slow
(my test of 
> a row with 30 00 columns (standard, regular) each with 8 bytes in name, and 40 bytes
in value, is about 16ms.
> this is all running in memory, no disk read is involved.
> through debugging we can find
> most of this time is spent on 
> [Wall Time]  org.apache.cassandra.db.Table.getRow(QueryFilter)
> [Wall Time]  org.apache.cassandra.db.ColumnFamilyStore.getColumnFamily(QueryFilter, ColumnFamily)
> [Wall Time]  org.apache.cassandra.db.ColumnFamilyStore.getColumnFamily(QueryFilter, int,
> [Wall Time]  org.apache.cassandra.db.ColumnFamilyStore.getTopLevelColumns(QueryFilter,
int, ColumnFamily)
> [Wall Time]  org.apache.cassandra.db.filter.QueryFilter.collectCollatedColumns(ColumnFamily,
Iterator, int)
> [Wall Time]  org.apache.cassandra.db.filter.SliceQueryFilter.collectReducedColumns(IColumnContainer,
Iterator, int)
> [Wall Time]  org.apache.cassandra.db.ColumnFamily.addColumn(IColumn)
> ColumnFamily.addColumn() is slow because it inserts into an internal concurrentSkipListMap()
that maps column names to values.
> this structure is slow for two reasons: it needs to do synchronization; it needs to maintain
a more complex structure of map.
> but if we look at the whole read path, thrift already defines the read output to be List<ColumnOrSuperColumn>
so it does not make sense to use a luxury map data structure in the interium and finally convert
it to a list. on the synchronization side, since the return CF is never going to be shared/modified
by other threads, we know the access is always single thread, so no synchronization is needed.
> but these 2 features are indeed needed for ColumnFamily in other cases, particularly
write. so we can provide a different ColumnFamily to CFS.getTopLevelColumnFamily(), so getTopLevelColumnFamily
no longer always creates the standard ColumnFamily, but take a provided returnCF, whose cost
is much cheaper.
> the provided patch is for demonstration now, will work further once we agree on the general
> CFS, ColumnFamily, and Table  are changed; a new FastColumnFamily is provided. the main
work is to let the FastColumnFamily use an array  for internal storage. at first I used binary
search to insert new columns in addColumn(), but later I found that even this is not necessary,
since all calling scenarios of ColumnFamily.addColumn() has an invariant that the inserted
columns come in sorted order (I still have an issue to resolve descending or ascending  now,
but ascending works). so the current logic is simply to compare the new column against the
end column in the array, if names not equal, append, if equal, reconcile.
> slight temporary hacks are made on getTopLevelColumnFamily so we have 2 flavors of the
method, one accepting a returnCF. but we could definitely think about what is the better way
to provide this returnCF.
> this patch compiles fine, no tests are provided yet. but I tested it in my application,
and the performance improvement is dramatic: it offers about 50% reduction in read time in
the 3000-column case.
> thanks
> Yang

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