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https://issues.apache.org/jira/browse/HBASE-13082?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14331989#comment-14331989
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stack commented on HBASE-13082:
-------------------------------
How does it work?
We create a RegionScanner per Scan. If we want to change the files that make up
a scan, we need to synchronize on 'this'. Calls to next, close, etc., have
synchronized on 'this' so the change of files can only be done when not nexting
or closing?
There may be many outstanding region scans going on. For the completion of
file updates to complete, all outstanding scanners will need to reach a 'safe
point', i.e. post a next or close call (I suppose close don't count ... so just
next).... A big row could take a while to return...
Sounds good to me.
Its kinda dirty passing lock from RegionScanner down from high level for use at
StoreScanner scope but hey, whatever works. Why were we not able to do this at
the StoreScanner scope again?
On compaction and flush being delayed, for flush, we will be in the commit
phase when we are trying to swap in the new file -- post flush from memory but
the snapshot of the memstore is still around and being read from -- so we could
hold on to memory pressure a little longer. For compaction, yeah, could be
reading from many files for a while longer though the compaction finished.
You done any testing. I like the bit where we remove the locks at Store level.
> Coarsen StoreScanner locks to RegionScanner
> -------------------------------------------
>
> Key: HBASE-13082
> URL: https://issues.apache.org/jira/browse/HBASE-13082
> Project: HBase
> Issue Type: Bug
> Reporter: Lars Hofhansl
> Attachments: 13082.txt
>
>
> Continuing where HBASE-10015 left of.
> We can avoid locking (and memory fencing) inside StoreScanner by deferring to
> the lock already held by the RegionScanner.
> In tests this shows quite a scan improvement and reduced CPU (the fences make
> the cores wait for memory fetches).
> There are some drawbacks too:
> * All calls to RegionScanner need to be remain synchronized
> * Implementors of coprocessors need to be diligent in following the locking
> contract. For example Phoenix does not lock RegionScanner.nextRaw() and
> required in the documentation (not picking on Phoenix, this one is my fault
> as I told them it's OK)
> * possible starving of flushes and compaction with heavy read load.
> RegionScanner operations would keep getting the locks and the
> flushes/compactions would not be able finalize the set of files.
> I'll have a patch soon.
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