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https://issues.apache.org/jira/browse/HIVE-3992?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13573402#comment-13573402
]
Gopal V commented on HIVE-3992:
-------------------------------
Testing dummy query (to simulate a "col in (select ...)" style query) at
SCALE=10
select /*+MAPJOIN(time_dim)*/ store_sales_rc.ss_item_sk from store_sales_rc
join time_dim on (store_sales_rc.ss_sold_time_sk = time_dim.t_time_sk) limit
100;
Before
{code}
2013-02-07 06:32:02,164 Stage-1 map = 0%, reduce = 0%
2013-02-07 06:32:20,082 Stage-1 map = 50%, reduce = 0%, Cumulative CPU 53.9 sec
2013-02-07 06:32:21,127 Stage-1 map = 100%, reduce = 0%, Cumulative CPU 61.59
sec
Job 0: Map: 8 Cumulative CPU: 61.59 sec HDFS Read: 104763092 HDFS Write:
4749 SUCCESS
Total MapReduce CPU Time Spent: 1 minutes 1 seconds 590 msec
Time taken: 34.572 seconds, Fetched: 100 row(s)
{code}
After
{code}
2013-02-07 06:35:29,413 Stage-1 map = 0%, reduce = 0%
2013-02-07 06:35:43,200 Stage-1 map = 25%, reduce = 0%, Cumulative CPU 9.31 sec
2013-02-07 06:35:44,247 Stage-1 map = 100%, reduce = 0%, Cumulative CPU 39.45
sec
MapReduce Total cumulative CPU time: 39 seconds 450 msec
Ended Job = job_1359695160319_0164
MapReduce Jobs Launched:
Job 0: Map: 8 Cumulative CPU: 39.45 sec HDFS Read: 25416952 HDFS Write:
4749 SUCCESS
Total MapReduce CPU Time Spent: 39 seconds 450 msec
Time taken: 31.351 seconds, Fetched: 100 row(s)
{code}
Now the interesting bit is that even though we cut down the CPU cost by almost
50%, the over-all latency drops only by 2 secs.
> Hive RCFile::sync(long) does a sub-sequence linear search for sync blocks
> -------------------------------------------------------------------------
>
> Key: HIVE-3992
> URL: https://issues.apache.org/jira/browse/HIVE-3992
> Project: Hive
> Issue Type: Bug
> Environment: Ubuntu x86_64/java-1.6/hadoop-2.0.3
> Reporter: Gopal V
> Attachments: HIVE-3992.patch, select-join-limit.html
>
>
> The following function does some bad I/O
> {code}
> public synchronized void sync(long position) throws IOException {
> ...
> try {
> seek(position + 4); // skip escape
> in.readFully(syncCheck);
> int syncLen = sync.length;
> for (int i = 0; in.getPos() < end; i++) {
> int j = 0;
> for (; j < syncLen; j++) {
> if (sync[j] != syncCheck[(i + j) % syncLen]) {
> break;
> }
> }
> if (j == syncLen) {
> in.seek(in.getPos() - SYNC_SIZE); // position before
> // sync
> return;
> }
> syncCheck[i % syncLen] = in.readByte();
> }
> }
> ...
> }
> {code}
> This causes a rather large number of readByte() calls which are passed onto a
> ByteBuffer via a single byte array.
> This results in rather a large amount of CPU being burnt in a the linear
> search for the sync pattern in the input RCFile (upto 92% for a skewed
> example - a trivial map-join + limit 100).
> This behaviour should be avoided at best or at least replaced by a rolling
> hash for efficient comparison, since it has a known byte-width of 16 bytes.
> Attached the stack trace from a Yourkit profile.
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