Cassandra DataStax driver slow?

我刚刚开始用Cassandra做实验，我使用的是c#和DataStax驱动程序(v3.0.8)。我想做一些性能测试，看看Cassandra处理时间序列数据的速度有多快。

结果令人窒息，因为它需要一个永恒的SELECT。所以我想我做错了什么。

我已经在我的本地计算机上安装了Cassandra，并创建了一个表:

CREATE KEYSPACE dm WITH replication = {'class': 'SimpleStrategy', 'replication_factor': '1'}  AND durable_writes = true;
CREATE TABLE dm.daily_data_by_day (
    symbol text,
    value_type int,
    as_of_day date,
    revision_timestamp_utc timestamp,
    value decimal,
    PRIMARY KEY ((symbol, value_type), as_of_day, revision_timestamp_utc)
) WITH CLUSTERING ORDER BY (as_of_day ASC, revision_timestamp_utc ASC)
    AND bloom_filter_fp_chance = 0.01
    AND caching = {'keys': 'ALL', 'rows_per_partition': 'NONE'}
    AND comment = ''
    AND compaction = {'class': 'org.apache.cassandra.db.compaction.SizeTieredCompactionStrategy', 'max_threshold': '32', 'min_threshold': '4'}
    AND compression = {'chunk_length_in_kb': '64', 'class': 'org.apache.cassandra.io.compress.LZ4Compressor'}
    AND crc_check_chance = 1.0
    AND dclocal_read_repair_chance = 0.1
    AND default_time_to_live = 0
    AND gc_grace_seconds = 864000
    AND max_index_interval = 2048
    AND memtable_flush_period_in_ms = 0
    AND min_index_interval = 128
    AND read_repair_chance = 0.0
    AND speculative_retry = '99PERCENTILE';

我在这个表中填充了大约1500万行，分为大约10000个分区，每个分区最多包含10000行。

下面是我正在运行的测试(根据phact的请求更新):

[Test]
public void SelectPerformance()
{
    _cluster = Cluster.Builder().AddContactPoint("127.0.0.1").Build();
    _stopwatch = new Stopwatch();
    var items = new[]
        {
            // 20 different items...
        };
    foreach (var item in items)
    {
        var watch = Stopwatch.StartNew();
        var rows = ExecuteQuery(item.Symbol, item.FieldType, item.StartDate, item.EndDate);
        watch.Stop();
        Console.WriteLine($"{watch.ElapsedMilliseconds}'t{rows.Length}");
    }
    Console.WriteLine($"Average Execute: {_stopwatch.ElapsedMilliseconds/items.Length}");
    _cluster.Dispose();
}
private Row[] ExecuteQuery(string symbol, int fieldType, LocalDate startDate, LocalDate endDate)
{
    using (var session = _cluster.Connect("dm"))
    {
        var ps = session.Prepare(
@"SELECT
    symbol,
    value_type,
    as_of_day,
    revision_timestamp_utc,
    value
FROM
    daily_data_by_day
WHERE
    symbol = ? AND
    value_type = ? AND
    as_of_day >= ? AND as_of_day < ?");
        var statement = ps.Bind(symbol, fieldType, startDate, endDate);
        statement.EnableTracing();
        _stopwatch.Start();
        var rowSet = session.Execute(statement);
        _stopwatch.Stop();
        return rowSet.ToArray();
    }
}

秒表告诉我，session.Execute()需要20-30毫秒来执行(更新:在更改代码以创建集群之后，只有一次我减少到大约15毫秒)。所以我启用了一些跟踪，得到了以下结果:

 activity                                                                  | source_elapsed 
--------------------------------------------------------------------------------------------
 Parsing SELECT symbol, value_type, as_of_day, revision_timestamp_utc,...; |             47 
                                                       Preparing statement |             98 
                     Executing single-partition query on daily_data_by_day |            922 
                                              Acquiring sstable references |            939 
 Skipped 0/5 non-slice-intersecting sstables, included 0 due to tombstones |            978 
                                   Bloom filter allows skipping sstable 74 |           1003 
                                   Bloom filter allows skipping sstable 75 |           1015 
                                   Bloom filter allows skipping sstable 72 |           1024 
                                   Bloom filter allows skipping sstable 73 |           1032 
                                              Key cache hit for sstable 63 |           1043 
                                 Merged data from memtables and 5 sstables |           1329 
                                       Read 100 live and 0 tombstone cells |           1353 

如果我正确理解了这个跟踪，Cassandra执行我的查询花费不到1.4毫秒。那么，其余时间DataStax驱动程序在做什么呢?

(作为参考，我对本地SQL Server实例进行了相同的性能测试，结果从c#执行相同的查询大约需要1-2毫秒)

更新:

我试图做一些分析，这不是那么容易做的异步代码，你不拥有…

我的结论是大部分时间都花在解析响应上。每个响应包含2000 - 3000行，解析每个响应大约需要9毫秒。反序列化花费的时间最多，大约为6.5 ms，十进制的时间最差，每个字段大约为3 ms。其他字段(text, int, date和timestamp)每个字段大约占用0.5 ms。

看看我测量的时间，我应该怀疑这一点:响应中的行越多，所需的时间就越长，而且几乎是线性的。