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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-4229) BatchWriter Locator cache out-of-sync when shared with tserver
Date Thu, 28 Apr 2016 20:38:13 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-4229?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15262940#comment-15262940
] 

ASF GitHub Bot commented on ACCUMULO-4229:
------------------------------------------

Github user keith-turner commented on the pull request:

    https://github.com/apache/accumulo/pull/96#issuecomment-215555038
  
    > The original reason why the locators cache is cleared every hour is to remove TabletLocators
that refer to tablets that don't exist or are rarely accessed, ya?
    
    The clearing was added to clean tablet location cache memory populated while processing
bulk imports. If tablet servers were not processing bulk imports, then only metadata table
tablet locations would be cached.
    
    >A WeakHashMap wouldn't work because it uses weak references for the keys
    
    oh yeah, I was thinking it was based on values... but its not.
    
    > What do you think of switching locators to a Guava cache with a 1 hour timeout on
entries since their last access?
    
    May be good.  Wouldn't we still need to prevent the guava cache from removing tablet locators
that are referenced outside the cache?  I looked at the Gauva cache javadoc and saw it supports
weakValues() and softValues() based caches.  Based on past experience (that I just remembered)
I am thinking weak references may be bad, it seems like weak references are collected aggressively
(when only referenced by the cache).  Clearing the cache too aggressively would not be good
for performance  SoftReferences would not suffer from the aggressive collection problem, but
soft refs have their own problems.
    



> BatchWriter Locator cache out-of-sync when shared with tserver
> --------------------------------------------------------------
>
>                 Key: ACCUMULO-4229
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-4229
>             Project: Accumulo
>          Issue Type: Bug
>          Components: client
>    Affects Versions: 1.6.5, 1.7.1
>            Reporter: Dylan Hutchison
>            Assignee: Dylan Hutchison
>
> BatchWriters that run a long time have write rates that sometimes mysteriously decrease
after the table it is writing to goes through a major compaction or a split.  The decrease
can be as bad as reducing throughput to 0.
> This was first first mentioned in this [email thread|https://mail-archives.apache.org/mod_mbox/accumulo-user/201406.mbox/%3CCAMz+DuvmmHegOn9EJeHR9H_rRpP50L2QZ53BbdruVO0pirArQw@mail.gmail.com%3E]
for major compactions.  
> I discovered this in this [email thread|https://mail-archives.apache.org/mod_mbox/accumulo-dev/201604.mbox/%3CCAPx%3DJkaY7fVh-U0O%2Bysx2d98LOGMcA4oEQOYgoPxR-0em4hdvg%40mail.gmail.com%3E]
for splits.  See the thread for some log messages.
> I turned on TRACE logs and I think I pinned it down: the TabletLocator cached by a BatchWriter
gets out of sync with the static cache of TabletLocators.
> # The TabletServerBatchWriter caches a TabletLocator from the static collection of TabletLocators
when it starts writing.  Suppose it is writing to tablet T1.
> # The TabletServerBatchWriter uses its locally cached TabletLocator inside its `binMutations`
method for its entire lifespan; this cache is never refreshed or updated to sync up with the
static collection of TabletLocators.
> # Every hour, the static collection of TabletLocators clears itself.  The next call to
get a TabletLocator from the static collection allocates a new TabletLocator.  Unfortunately,
the TabletServerBatchWriter does not reflect this change and continues to use the old, locally
cached TabletLocator.
> # Tablet T1 splits into T2 and T3, which closes T1.  As such, it no longer exists and
the tablet server that receives the entries meant to go to T1 all fail to write because T1
is closed.
> # The TabletServerBatchWriter receives the response from the tablet server that all entries
failed to write.  It invalidates the cache of the *new* TabletLocator obtained from the static
collection of TabletLocators.  The old TabletLocator that is cached locally does not get invalidated.
> # The TabletServerBatchWriter re-queues the failed entries and tries to write them to
the same closed tablet T1, because it is still looking up tablets using the old TabletLocator.
> This behavior subsumes the circumstances William wrote about in the thread he mentioned.
 The problem would occur as a result of either splits or major compactions.  It would only
stop the BatchWriter if its entire memory filled up with writes to the same tablet that was
closed as a result of a majc or split; otherwise it would just slow down the BatchWriter by
failing to write some number of entries with every RPC.
> There are a few solutions we can think of.  
> # Not have the MutationWriter inside the TabletServerBatchWriter locally cache TabletLocators.
 I suspect this was done for performance reasons, so it's probably not a good solution. 
> # Have all the MutationWriters clear their cache at the same time the static TabletLocator
cache clears.  We could store a reference to the Map that each MutationWriter has inside a
static synchronized WeakHashMap.  The only time the weak map needs to be accessed is:
> ## When a MutationWriter is constructed (from constructing a TabletServerBatchWriter),
add its new local TabletLocator cache to the weak map.
> ## When the static TabletLocator cache is cleared, also clear every map in the weak map.
> # Another solution is to make the invalidate calls on the local TabletLocator cache rather
than the global static one.  If we go this route we should double check the idea to make sure
it does not impact the correctness of any other pieces of code that use the cache. 
> # Perhaps the simplest solution is to put an extra Boolean variable inside the Locators
indicating whether they are valid.  When they are cleared, their Boolean variables set to
false.  Before a client uses a locator from cache, it checks its Boolean indicator.
> The TimeoutTabletLocator does not help when no timeout is set on the BatchWriter (the
default behavior).



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