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https://issues.apache.org/jira/browse/CASSANDRA-2843?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Sylvain Lebresne updated CASSANDRA-2843:
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Attachment: 2843.patch
I do think that using a specific implementation for the backing map of a
ColumnFamily during is a good idea. It is clear that avoiding synchronization
will be faster, and given the type of operations we do during reads (insertion
in sorted order and iteration), an ArrayList backed solution is sure to be
faster too. I will also be much gentle on the GC that the linked list
ConcurrentSkipListMap uses. I think that all those will help even with
relatively small reads. So let's focus on that for this ticket and let other
potential improvement to other ticket, especially if it is unclear they bear
any noticeable speedup.
So focusing on the patch itself:
* We really shouldn't "simply" extend ColumnFamily as the patch does. This is
quite frankly ugly and will be a maintenance nightmare (you'll have to check
you did overwrite every function that touch the map (which is not the case in
the patch) and every update to ColumnFamily have to be aware that it should
update FastColumnFamily as well).
* The implementation of FastColumnFamily should really be a fully functionnal
ColumnFamily implementation (albeit not synchronized). That is, we can't assume
that addition will always be in strict increasing order, otherwise again this
will be too hard to use.
* The addAll function can be optimized given that both input are sorted.
Granted, I don't think it is used in the read path, but I think that the new
ColumnFamily implementation could advantageously be used during compaction (by
preCompactedRow typically, and possibly other places where concurrent access is
not an issue) where this would matter.
Attaching a version of the patch (2843.patch) that tries to address all the
remarks above. The patch is against trunk (not 0.8 branch), because it build on
the recently committed refactor of ColumnFamily. It refactors ColumnFamily
(AbstractColumnContainer actually) to allow for a pluggable backing column map.
The ConcurrentSkipListMap implemn is name ThreadSafeColumnMap and the new one
is called ArrayBackedColumnMap (which I prefer to FastSomething since it's not
a very helpful name).
On the ColumnFamilyStore side, instead of feeding the returnCF to
getTopLevelColumns, I pass along a factory (that each backing implementation
provides). The main goal was to avoid creating a columnFamily when it's useless
(if row cache is enabled on the CF -- btw, this ticket only improve on read for
column family with no cache).
Micro benchmarks does show that on the operation involved during a read
(addition of column + iteration), the ArrayBacked implementation is faster than
the ConcurrentSkipListMap based one. Interestingly though, this is mainly true
when some reconciliation of columns happens. That is, if you only add columns
with different names, the ArrayBacked implementation is faster, but not
dramatically so. If you start adding column that have to be resolved, the
ArrayBacked implementation becomes much faster, even with a reasonably small
number of columns (inserting 100 columns with only 10 unique column names, the
ArrayBacked is already >30% faster). And this mostly due to the overhead of
synchronization (of replace()): a TreeMap based implementation is slightly
slower than the ArrayBacked one but not by a lot and thus is much faster than
the ConcurrentSkipListMap implementation.
The attached patch should be ready for review (though it could probably use a
few unit test for the new ArrayBacked implementation).
> better performance on long row read
> -----------------------------------
>
> Key: CASSANDRA-2843
> URL: https://issues.apache.org/jira/browse/CASSANDRA-2843
> Project: Cassandra
> Issue Type: New Feature
> Reporter: Yang Yang
> Attachments: 2843.patch, fast_cf_081_trunk.diff
>
>
> 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,
> ColumnFamily)
> [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 direction.
> 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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