[jira] [Commented] (HBASE-21418) Reduce a number of reseek operations in MemstoreScanner when seek point is close to the current row.
[
https://issues.apache.org/jira/browse/HBASE-21418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=16835033#comment-16835033
]
HBase QA commented on HBASE-21418:
--
| (x) *{color:red}-1 overall{color}* |
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[jira] [Commented] (HBASE-21418) Reduce a number of reseek operations in MemstoreScanner when seek point is close to the current row.
[ https://issues.apache.org/jira/browse/HBASE-21418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=16684460#comment-16684460 ] Lars Hofhansl commented on HBASE-21418: --- I'll look this evening (bit busy throughout the day) > Reduce a number of reseek operations in MemstoreScanner when seek point is > close to the current row. > > > Key: HBASE-21418 > URL: https://issues.apache.org/jira/browse/HBASE-21418 > Project: HBase > Issue Type: Improvement > Components: scan, Scanners >Affects Versions: 1.2.5 >Reporter: Jeongdae Kim >Assignee: Jeongdae Kim >Priority: Minor > Labels: performance > Attachments: HBASE-21418.branch-1.2.001.patch, > HBASE-21418.branch-1.2.001.patch, HBASE-21418.branch-1.2.002.patch > > > We observed “responseTooSlow” logs for Get requests in our production > clusters. even some get requests were responded after 10 seconds. > Affected get requests were done with the timerange, and target rows have many > columns that have some versions. > We reproduced this issue, and found this behavior happens only when scanning > in the memstore. after flushing the HStore, this slow response issue for Get > disappeared and all same get requests are responded very quickly. > > We investigated this case, and found this performance difference between > memstore scanner and hfile scanner is caused by the number of reseek > operations executed while scanning. When a store scanner needs to reseek the > next column, Hfile scanner wisely decide whether it have to reseek or not by > checking the seek point is in current block, whereas memstore scanner just do > reseek without decision unlike Hfile scanner. In our case, almost all columns > in the memstore have older timestamp than scan(get)’s timerange, and so many > reseek operations occur as much as about the number of columns. This results > in increasing the response time of Get requests sporadically. > > To improve the reseek operation of the memstore scanner, i think it’s better > skipping than seeking when reseek requested, if seek point is quite close to > current cell that the scanner is pointing now.(Actually, i changed > MatchCode.SEEK_NEXT_COL to MatchCode.Skip in our case, and the response time > of Get was 6x faster than before) But we can’t decide whether seek point is > close to the current cell or not, because memstore scannner has no > information such as next block index. > Before HBASE-13109, Scan.HINT_LOOKAHEAD was introduced to handle like this > case, and it may be deprecated someday. But, i think that hint is still be > useful for the memstore scanner to try to skip first, before reseeking, and > with this option we can make reseek operations of memstore scanner smarter. > > I tested this patch in our case, and got the same result as i changed > matchcode (mentioned above). -- This message was sent by Atlassian JIRA (v7.6.3#76005)
[jira] [Commented] (HBASE-21418) Reduce a number of reseek operations in MemstoreScanner when seek point is close to the current row.
[
https://issues.apache.org/jira/browse/HBASE-21418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=16681314#comment-16681314
]
Hadoop QA commented on HBASE-21418:
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[jira] [Commented] (HBASE-21418) Reduce a number of reseek operations in MemstoreScanner when seek point is close to the current row.
[ https://issues.apache.org/jira/browse/HBASE-21418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=16681082#comment-16681082 ] Jeongdae Kim commented on HBASE-21418: -- In the second patch, I tried to collect stats of columns with a bit of cost(stats collected only when flushing or compacting, because there are already a lot of comparisions in there), and use it to decide whether doing reseek or skip, when reseeking to next row, and use maxVersions when reseeking to next column as a heuristic. Please take a look again [~lhofhansl]. Thanks. > Reduce a number of reseek operations in MemstoreScanner when seek point is > close to the current row. > > > Key: HBASE-21418 > URL: https://issues.apache.org/jira/browse/HBASE-21418 > Project: HBase > Issue Type: Improvement > Components: scan, Scanners >Affects Versions: 1.2.5 >Reporter: Jeongdae Kim >Assignee: Jeongdae Kim >Priority: Minor > Labels: performance > Attachments: HBASE-21418.branch-1.2.001.patch, > HBASE-21418.branch-1.2.001.patch, HBASE-21418.branch-1.2.002.patch > > > We observed “responseTooSlow” logs for Get requests in our production > clusters. even some get requests were responded after 10 seconds. > Affected get requests were done with the timerange, and target rows have many > columns that have some versions. > We reproduced this issue, and found this behavior happens only when scanning > in the memstore. after flushing the HStore, this slow response issue for Get > disappeared and all same get requests are responded very quickly. > > We investigated this case, and found this performance difference between > memstore scanner and hfile scanner is caused by the number of reseek > operations executed while scanning. When a store scanner needs to reseek the > next column, Hfile scanner wisely decide whether it have to reseek or not by > checking the seek point is in current block, whereas memstore scanner just do > reseek without decision unlike Hfile scanner. In our case, almost all columns > in the memstore have older timestamp than scan(get)’s timerange, and so many > reseek operations occur as much as about the number of columns. This results > in increasing the response time of Get requests sporadically. > > To improve the reseek operation of the memstore scanner, i think it’s better > skipping than seeking when reseek requested, if seek point is quite close to > current cell that the scanner is pointing now.(Actually, i changed > MatchCode.SEEK_NEXT_COL to MatchCode.Skip in our case, and the response time > of Get was 6x faster than before) But we can’t decide whether seek point is > close to the current cell or not, because memstore scannner has no > information such as next block index. > Before HBASE-13109, Scan.HINT_LOOKAHEAD was introduced to handle like this > case, and it may be deprecated someday. But, i think that hint is still be > useful for the memstore scanner to try to skip first, before reseeking, and > with this option we can make reseek operations of memstore scanner smarter. > > I tested this patch in our case, and got the same result as i changed > matchcode (mentioned above). -- This message was sent by Atlassian JIRA (v7.6.3#76005)
[jira] [Commented] (HBASE-21418) Reduce a number of reseek operations in MemstoreScanner when seek point is close to the current row.
[
https://issues.apache.org/jira/browse/HBASE-21418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=16672938#comment-16672938
]
Jeongdae Kim commented on HBASE-21418:
--
Thanks for your comments. I’ll reflect your comments to the next patch.
{quote}
Generally I am not a fan of adding more HBase and/or scan options that one has
to know about. (which is why I had removed the LOOK_AHEAD hint that I myself
had added a bit earlier).
{quote}
I 100% agree with you, and would like to do without options too. but, I
couldn’t find a nice solution without extra cost.
{quote}
Why max versions here? The SEEKing can also be an issue with many columns,
right?
If we can, let's find a heuristic to do this automatically (like I did with
HFiles), so that a user won't have to hint.
{quote}
Right, I used the max versions as a heuristic in case that users pass no hint.
I had no any idea about proper heuristic.
If we can bear small extra costs when putting cells into a memstore, What about
maintaining some stats for columns and using it to decide whether doing seek
operations or not. Let me try to make a patch for this.
> Reduce a number of reseek operations in MemstoreScanner when seek point is
> close to the current row.
>
>
> Key: HBASE-21418
> URL: https://issues.apache.org/jira/browse/HBASE-21418
> Project: HBase
> Issue Type: Improvement
> Components: scan, Scanners
>Affects Versions: 1.2.5
>Reporter: Jeongdae Kim
>Assignee: Jeongdae Kim
>Priority: Minor
> Labels: performance
> Attachments: HBASE-21418.branch-1.2.001.patch,
> HBASE-21418.branch-1.2.001.patch
>
>
> We observed “responseTooSlow” logs for Get requests in our production
> clusters. even some get requests were responded after 10 seconds.
> Affected get requests were done with the timerange, and target rows have many
> columns that have some versions.
> We reproduced this issue, and found this behavior happens only when scanning
> in the memstore. after flushing the HStore, this slow response issue for Get
> disappeared and all same get requests are responded very quickly.
>
> We investigated this case, and found this performance difference between
> memstore scanner and hfile scanner is caused by the number of reseek
> operations executed while scanning. When a store scanner needs to reseek the
> next column, Hfile scanner wisely decide whether it have to reseek or not by
> checking the seek point is in current block, whereas memstore scanner just do
> reseek without decision unlike Hfile scanner. In our case, almost all columns
> in the memstore have older timestamp than scan(get)’s timerange, and so many
> reseek operations occur as much as about the number of columns. This results
> in increasing the response time of Get requests sporadically.
>
> To improve the reseek operation of the memstore scanner, i think it’s better
> skipping than seeking when reseek requested, if seek point is quite close to
> current cell that the scanner is pointing now.(Actually, i changed
> MatchCode.SEEK_NEXT_COL to MatchCode.Skip in our case, and the response time
> of Get was 6x faster than before) But we can’t decide whether seek point is
> close to the current cell or not, because memstore scannner has no
> information such as next block index.
> Before HBASE-13109, Scan.HINT_LOOKAHEAD was introduced to handle like this
> case, and it may be deprecated someday. But, i think that hint is still be
> useful for the memstore scanner to try to skip first, before reseeking, and
> with this option we can make reseek operations of memstore scanner smarter.
>
> I tested this patch in our case, and got the same result as i changed
> matchcode (mentioned above).
--
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[jira] [Commented] (HBASE-21418) Reduce a number of reseek operations in MemstoreScanner when seek point is close to the current row.
[
https://issues.apache.org/jira/browse/HBASE-21418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=16672936#comment-16672936
]
Jeongdae Kim commented on HBASE-21418:
--
Thanks for the comment [~yuzhih...@gmail.com].
{quote}
What is TestLookAheadBeforeReseek supposed to show without the fix ?
{quote}
It's for showing performance difference . I'll remove the test from my patch
and make an external link for this test.
> Reduce a number of reseek operations in MemstoreScanner when seek point is
> close to the current row.
>
>
> Key: HBASE-21418
> URL: https://issues.apache.org/jira/browse/HBASE-21418
> Project: HBase
> Issue Type: Improvement
> Components: scan, Scanners
>Affects Versions: 1.2.5
>Reporter: Jeongdae Kim
>Assignee: Jeongdae Kim
>Priority: Minor
> Labels: performance
> Attachments: HBASE-21418.branch-1.2.001.patch,
> HBASE-21418.branch-1.2.001.patch
>
>
> We observed “responseTooSlow” logs for Get requests in our production
> clusters. even some get requests were responded after 10 seconds.
> Affected get requests were done with the timerange, and target rows have many
> columns that have some versions.
> We reproduced this issue, and found this behavior happens only when scanning
> in the memstore. after flushing the HStore, this slow response issue for Get
> disappeared and all same get requests are responded very quickly.
>
> We investigated this case, and found this performance difference between
> memstore scanner and hfile scanner is caused by the number of reseek
> operations executed while scanning. When a store scanner needs to reseek the
> next column, Hfile scanner wisely decide whether it have to reseek or not by
> checking the seek point is in current block, whereas memstore scanner just do
> reseek without decision unlike Hfile scanner. In our case, almost all columns
> in the memstore have older timestamp than scan(get)’s timerange, and so many
> reseek operations occur as much as about the number of columns. This results
> in increasing the response time of Get requests sporadically.
>
> To improve the reseek operation of the memstore scanner, i think it’s better
> skipping than seeking when reseek requested, if seek point is quite close to
> current cell that the scanner is pointing now.(Actually, i changed
> MatchCode.SEEK_NEXT_COL to MatchCode.Skip in our case, and the response time
> of Get was 6x faster than before) But we can’t decide whether seek point is
> close to the current cell or not, because memstore scannner has no
> information such as next block index.
> Before HBASE-13109, Scan.HINT_LOOKAHEAD was introduced to handle like this
> case, and it may be deprecated someday. But, i think that hint is still be
> useful for the memstore scanner to try to skip first, before reseeking, and
> with this option we can make reseek operations of memstore scanner smarter.
>
> I tested this patch in our case, and got the same result as i changed
> matchcode (mentioned above).
--
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[jira] [Commented] (HBASE-21418) Reduce a number of reseek operations in MemstoreScanner when seek point is close to the current row.
[
https://issues.apache.org/jira/browse/HBASE-21418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=16672052#comment-16672052
]
Hadoop QA commented on HBASE-21418:
---
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16s{color} | {color:red} hbase-server: The patch generated 13 new + 164
unchanged - 1 fixed = 177 total (was 165) {color} |
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{color:black} {color} |
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[jira] [Commented] (HBASE-21418) Reduce a number of reseek operations in MemstoreScanner when seek point is close to the current row.
[
https://issues.apache.org/jira/browse/HBASE-21418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=16671944#comment-16671944
]
Lars Hofhansl commented on HBASE-21418:
---
Thanks for looking into this [~Jeongdae Kim]!
Generally I am not a fan of adding more HBase and/or scan options that one has
to know about. (which is why I had removed the LOOK_AHEAD hint that I myself
had added a bit earlier).
Is there no way to guess whether we should SKIP or SEEK?
Patch looks good in general. Few comments:
{code:java}
- /**
- * @deprecated without replacement
- * This is now a no-op, SEEKs and SKIPs are optimizated
automatically.
- * Will be removed in 2.0+
- */
- @Deprecated
+
public static final String HINT_LOOKAHEAD = "_look_ahead_";{code}
We might want to put the older (EXPERT_ONLY) comment back (or any comment
explaining what this is doing).
{code:java}
+MemStoreScanner(long readPoint, ScanQueryMatcher scanQueryMatcher) {
+ this(readPoint);
+
+ if (scanQueryMatcher != null) {
+int lookAheadRows =
conf.getInt("hbase.hregion.memstore.scanner.lookahead.rows", 0);
+if (scanQueryMatcher.getLookAheadBeforeReseek() <= lookAheadRows) {
+ this.lookAheadRows = scanQueryMatcher.getLookAheadBeforeReseek();
+}
+ }
+}
+{code}
Note that conf.getXXX can be slow itself. We might want to cache this value
somewhere.
{code:java}
+
+ if (lookAheadRows > 0 && peek() != null) {
+for (int i = lookAheadRows; i > 0; --i) {
+ next();
+ if (peek() == null) {
+return false;
+ }
+ if (comparator.compare(peek(), key) >= 0) {
+return true;
+ }
+}
+ }
+{code}
This is a nit, but since the value if {{i}} is not actually used inside the
loop, I'd prefer
{{for (int i=0; i< lookAheadRows; i++)...}}
{code:java}
+
+byte[] attr = scan.getAttribute(Scan.HINT_LOOKAHEAD);
+this.lookAheadBeforeSeek = Math.min(attr == null ? 0 : Bytes.toInt(attr),
scanInfo.getMaxVersions());
+{code}
Why max versions here? The SEEKing can also be an issue with many columns,
right?
If we can, let's find a heuristic to do this automatically (like I did with
HFiles), so that a user won't have to hint.
> Reduce a number of reseek operations in MemstoreScanner when seek point is
> close to the current row.
>
>
> Key: HBASE-21418
> URL: https://issues.apache.org/jira/browse/HBASE-21418
> Project: HBase
> Issue Type: Improvement
> Components: scan, Scanners
>Affects Versions: 1.2.5
>Reporter: Jeongdae Kim
>Assignee: Jeongdae Kim
>Priority: Minor
> Labels: performance
> Attachments: HBASE-21418.branch-1.2.001.patch,
> HBASE-21418.branch-1.2.001.patch
>
>
> We observed “responseTooSlow” logs for Get requests in our production
> clusters. even some get requests were responded after 10 seconds.
> Affected get requests were done with the timerange, and target rows have many
> columns that have some versions.
> We reproduced this issue, and found this behavior happens only when scanning
> in the memstore. after flushing the HStore, this slow response issue for Get
> disappeared and all same get requests are responded very quickly.
>
> We investigated this case, and found this performance difference between
> memstore scanner and hfile scanner is caused by the number of reseek
> operations executed while scanning. When a store scanner needs to reseek the
> next column, Hfile scanner wisely decide whether it have to reseek or not by
> checking the seek point is in current block, whereas memstore scanner just do
> reseek without decision unlike Hfile scanner. In our case, almost all columns
> in the memstore have older timestamp than scan(get)’s timerange, and so many
> reseek operations occur as much as about the number of columns. This results
> in increasing the response time of Get requests sporadically.
>
> To improve the reseek operation of the memstore scanner, i think it’s better
> skipping than seeking when reseek requested, if seek point is quite close to
> current cell that the scanner is pointing now.(Actually, i changed
> MatchCode.SEEK_NEXT_COL to MatchCode.Skip in our case, and the response time
> of Get was 6x faster than before) But we can’t decide whether seek point is
> close to the current cell or not, because memstore scannner has no
> information such as next block index.
> Before HBASE-13109, Scan.HINT_LOOKAHEAD was introduced to handle like this
> case, and it may be deprecated someday. But, i think that hint is still be
> useful for the memstore scanner to try to skip first, before reseeking, and
> with this option we can make reseek
[jira] [Commented] (HBASE-21418) Reduce a number of reseek operations in MemstoreScanner when seek point is close to the current row.
[ https://issues.apache.org/jira/browse/HBASE-21418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=16671928#comment-16671928 ] Lars Hofhansl commented on HBASE-21418: --- (Sorry about the re-assign, hit some keyboard combination that did that) In the HFile case we only reseek if the seek would land us in a different HFile block. For the memstore we not have a similar metric. I'm suprised that reseek is so expensive in the memstore case. Taking a look now. > Reduce a number of reseek operations in MemstoreScanner when seek point is > close to the current row. > > > Key: HBASE-21418 > URL: https://issues.apache.org/jira/browse/HBASE-21418 > Project: HBase > Issue Type: Improvement > Components: scan, Scanners >Affects Versions: 1.2.5 >Reporter: Jeongdae Kim >Assignee: Jeongdae Kim >Priority: Minor > Labels: performance > Attachments: HBASE-21418.branch-1.2.001.patch, > HBASE-21418.branch-1.2.001.patch > > > We observed “responseTooSlow” logs for Get requests in our production > clusters. even some get requests were responded after 10 seconds. > Affected get requests were done with the timerange, and target rows have many > columns that have some versions. > We reproduced this issue, and found this behavior happens only when scanning > in the memstore. after flushing the HStore, this slow response issue for Get > disappeared and all same get requests are responded very quickly. > > We investigated this case, and found this performance difference between > memstore scanner and hfile scanner is caused by the number of reseek > operations executed while scanning. When a store scanner needs to reseek the > next column, Hfile scanner wisely decide whether it have to reseek or not by > checking the seek point is in current block, whereas memstore scanner just do > reseek without decision unlike Hfile scanner. In our case, almost all columns > in the memstore have older timestamp than scan(get)’s timerange, and so many > reseek operations occur as much as about the number of columns. This results > in increasing the response time of Get requests sporadically. > > To improve the reseek operation of the memstore scanner, i think it’s better > skipping than seeking when reseek requested, if seek point is quite close to > current cell that the scanner is pointing now.(Actually, i changed > MatchCode.SEEK_NEXT_COL to MatchCode.Skip in our case, and the response time > of Get was 6x faster than before) But we can’t decide whether seek point is > close to the current cell or not, because memstore scannner has no > information such as next block index. > Before HBASE-13109, Scan.HINT_LOOKAHEAD was introduced to handle like this > case, and it may be deprecated someday. But, i think that hint is still be > useful for the memstore scanner to try to skip first, before reseeking, and > with this option we can make reseek operations of memstore scanner smarter. > > I tested this patch in our case, and got the same result as i changed > matchcode (mentioned above). -- This message was sent by Atlassian JIRA (v7.6.3#76005)
[jira] [Commented] (HBASE-21418) Reduce a number of reseek operations in MemstoreScanner when seek point is close to the current row.
[
https://issues.apache.org/jira/browse/HBASE-21418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=16671750#comment-16671750
]
Ted Yu commented on HBASE-21418:
For the new test, I ran it without the rest of the patch:
{code}
Running org.apache.hadoop.hbase.client.TestLookAheadBeforeReseek
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 24.647 sec - in
org.apache.hadoop.hbase.client.TestLookAheadBeforeReseek
{code}
What is TestLookAheadBeforeReseek supposed to show without the fix ?
> Reduce a number of reseek operations in MemstoreScanner when seek point is
> close to the current row.
>
>
> Key: HBASE-21418
> URL: https://issues.apache.org/jira/browse/HBASE-21418
> Project: HBase
> Issue Type: Improvement
> Components: scan, Scanners
>Affects Versions: 1.2.5
>Reporter: Jeongdae Kim
>Assignee: Jeongdae Kim
>Priority: Minor
> Labels: performance
> Attachments: HBASE-21418.branch-1.2.001.patch
>
>
> We observed “responseTooSlow” logs for Get requests in our production
> clusters. even some get requests were responded after 10 seconds.
> Affected get requests were done with the timerange, and target rows have many
> columns that have some versions.
> We reproduced this issue, and found this behavior happens only when scanning
> in the memstore. after flushing the HStore, this slow response issue for Get
> disappeared and all same get requests are responded very quickly.
>
> We investigated this case, and found this performance difference between
> memstore scanner and hfile scanner is caused by the number of reseek
> operations executed while scanning. When a store scanner needs to reseek the
> next column, Hfile scanner wisely decide whether it have to reseek or not by
> checking the seek point is in current block, whereas memstore scanner just do
> reseek without decision unlike Hfile scanner. In our case, almost all columns
> in the memstore have older timestamp than scan(get)’s timerange, and so many
> reseek operations occur as much as about the number of columns. This results
> in increasing the response time of Get requests sporadically.
>
> To improve the reseek operation of the memstore scanner, i think it’s better
> skipping than seeking when reseek requested, if seek point is quite close to
> current cell that the scanner is pointing now.(Actually, i changed
> MatchCode.SEEK_NEXT_COL to MatchCode.Skip in our case, and the response time
> of Get was 6x faster than before) But we can’t decide whether seek point is
> close to the current cell or not, because memstore scannner has no
> information such as next block index.
> Before HBASE-13109, Scan.HINT_LOOKAHEAD was introduced to handle like this
> case, and it may be deprecated someday. But, i think that hint is still be
> useful for the memstore scanner to try to skip first, before reseeking, and
> with this option we can make reseek operations of memstore scanner smarter.
>
> I tested this patch in our case, and got the same result as i changed
> matchcode (mentioned above).
--
This message was sent by Atlassian JIRA
(v7.6.3#76005)
[jira] [Commented] (HBASE-21418) Reduce a number of reseek operations in MemstoreScanner when seek point is close to the current row.
[
https://issues.apache.org/jira/browse/HBASE-21418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=16671453#comment-16671453
]
Hadoop QA commented on HBASE-21418:
---
| (x) *{color:red}-1 overall{color}* |
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\\
|| Vote || Subsystem || Runtime || Comment ||
| {color:blue}0{color} | {color:blue} reexec {color} | {color:blue} 17m
44s{color} | {color:blue} Docker mode activated. {color} |
|| || || || {color:brown} Prechecks {color} ||
| {color:blue}0{color} | {color:blue} findbugs {color} | {color:blue} 0m
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|| || || || {color:brown} branch-1.2 Compile Tests {color} ||
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| {color:green}+1{color} | {color:green} javadoc {color} | {color:green} 0m
51s{color} | {color:green} branch-1.2 passed with JDK v1.7.0_191 {color} |
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|| || || || {color:brown} Other Tests {color} ||
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| {color:black}{color} | {color:black} {color} | {color:black}153m 32s{color} |
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|| Reason || Tests ||
|
[jira] [Commented] (HBASE-21418) Reduce a number of reseek operations in MemstoreScanner when seek point is close to the current row.
[ https://issues.apache.org/jira/browse/HBASE-21418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=16671239#comment-16671239 ] Jeongdae Kim commented on HBASE-21418: -- [~lhofhansl] Could you take a look? > Reduce a number of reseek operations in MemstoreScanner when seek point is > close to the current row. > > > Key: HBASE-21418 > URL: https://issues.apache.org/jira/browse/HBASE-21418 > Project: HBase > Issue Type: Improvement > Components: scan, Scanners >Affects Versions: 1.2.5 >Reporter: Jeongdae Kim >Assignee: Jeongdae Kim >Priority: Minor > Labels: performance > Attachments: HBASE-21418.branch-1.2.001.patch > > > We observed “responseTooSlow” logs for Get requests in our production > clusters. even some get requests were responded after 10 seconds. > Affected get requests were done with the timerange, and target rows have many > columns that have some versions. > We reproduced this issue, and found this behavior happens only when scanning > in the memstore. after flushing the HStore, this slow response issue for Get > disappeared and all same get requests are responded very quickly. > > We investigated this case, and found this performance difference between > memstore scanner and hfile scanner is caused by the number of reseek > operations executed while scanning. When a store scanner needs to reseek the > next column, Hfile scanner wisely decide whether it have to reseek or not by > checking the seek point is in current block, whereas memstore scanner just do > reseek without decision unlike Hfile scanner. In our case, almost all columns > in the memstore have older timestamp than scan(get)’s timerange, and so many > reseek operations occur as much as about the number of columns. This results > in increasing the response time of Get requests sporadically. > > To improve the reseek operation of the memstore scanner, i think it’s better > skipping than seeking when reseek requested, if seek point is quite close to > current cell that the scanner is pointing now.(Actually, i changed > MatchCode.SEEK_NEXT_COL to MatchCode.Skip in our case, and the response time > of Get was 6x faster than before) But we can’t decide whether seek point is > close to the current cell or not, because memstore scannner has no > information such as next block index. > Before HBASE-13109, Scan.HINT_LOOKAHEAD was introduced to handle like this > case, and it may be deprecated someday. But, i think that hint is still be > useful for the memstore scanner to try to skip first, before reseeking, and > with this option we can make reseek operations of memstore scanner smarter. > > I tested this patch in our case, and got the same result as i changed > matchcode (mentioned above). -- This message was sent by Atlassian JIRA (v7.6.3#76005)
