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From "Yang Yang (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (CASSANDRA-2843) better performance on long row read
Date Thu, 07 Jul 2011 08:30:16 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-2843?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Yang Yang updated CASSANDRA-2843:
---------------------------------

    Comment: was deleted

(was: Thanks Sylvain

Its great to see this taking shape

Im on vacation with a pbone so very cursory comments

" this implementation should not expect the input always come in already
sorted order" then probably it would be difficult to achieve a lot of per
gain easily. Mainly there are two areas of attack. Synch and cheaper data
structure. Actually during my tests I found that sync is not the main issue:
going from CSLM to treemap is actually slower. Your tests show different so
we'd better confirm. If I were correct in tests. Then that means not
assuming special circumstances would be like finding a generic silver bullet
which is very difficult
https://issues.apache.org/jira/browse/CASSANDRA-2843?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel]
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.
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).
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.
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.
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).
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).
(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.
use a few unit test for the new ArrayBacked implementation).
considerably slow (my test of
and 40 bytes in value, is about 16ms.
org.apache.cassandra.db.ColumnFamilyStore.getColumnFamily(QueryFilter,
ColumnFamily)
org.apache.cassandra.db.ColumnFamilyStore.getColumnFamily(QueryFilter, int,
ColumnFamily)
org.apache.cassandra.db.ColumnFamilyStore.getTopLevelColumns(QueryFilter,
int, ColumnFamily)
org.apache.cassandra.db.filter.QueryFilter.collectCollatedColumns(ColumnFamily,
Iterator, int)
org.apache.cassandra.db.filter.SliceQueryFilter.collectReducedColumns(IColumnContainer,
Iterator, int)
concurrentSkipListMap() that maps column names to values.
it needs to maintain a more complex structure of map.
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.
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.
agree on the general direction.
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.
flavors of the method, one accepting a returnCF. but we could definitely
think about what is the better way to provide this returnCF.
my application, and the performance improvement is dramatic: it offers about
50% reduction in read time in the 3000-column case.
)

> 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, microBenchmark.patch
>
>
> 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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