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Date: Wed, 16 Feb 2011 00:40:58 +0000 (UTC)
From: "Stu Hood (JIRA)" <jira@apache.org>
To: commits@cassandra.apache.org
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Subject: [jira] Issue Comment Edited: (CASSANDRA-1954) Double-check or
 replace RRW memtable lock
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    [ https://issues.apache.org/jira/browse/CASSANDRA-1954?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12995091#comment-12995091 ] 

Stu Hood edited comment on CASSANDRA-1954 at 2/16/11 12:39 AM:
---------------------------------------------------------------

An idea to replace this lock was proposed in IRC yesterday: if the list of memtables and sstables was stored in a single immutable, cas'able structure, you could atomically swap a memtable from active to flushing, and then from flushing into an sstable. Example structure:
{code:java}class View {
  final List<Memtable> active;
  final List<Memtable> flushing;
  final List<SSTableReader> sstables;
}{code}

So a writing thread noticing an active Memtable past its threshold would attempt to CAS in a new Memtable while moving the current memtable to flushing. A thread finishing a flush would attempt to CAS the memtable it flushed out of flushing and into the sstable list.

EDIT: Bah... this doesn't actually remove the necessity of the write lock, since we need to ensure that writers are not looking at a memtable that has been moved to flushing. We'd need another solution to that problem.

      was (Author: stuhood):
    An idea to replace this lock was proposed in IRC yesterday: if the list of memtables and sstables was stored in a single immutable, cas'able structure, you could atomically swap a memtable from active to flushing, and then from flushing into an sstable. Example structure:
{code:java}class View {
  final List<Memtable> active;
  final List<Memtable> flushing;
  final List<SSTableReader> sstables;
}{code}

So a writing thread noticing an active Memtable past its threshold would attempt to CAS in a new Memtable while moving the current memtable to flushing. A thread finishing a flush would attempt to CAS the memtable it flushed out of flushing and into the sstable list.
  
> Double-check or replace RRW memtable lock
> -----------------------------------------
>
>                 Key: CASSANDRA-1954
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-1954
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Stu Hood
>            Priority: Minor
>         Attachments: 0001-Double-check-in-maybeSwitchMemtable-to-minimize-writeL.txt
>
>
> {quote}...when a Memtable reaches its threshold, up to (all) N write threads will often notice, and race to acquire the writeLock in order to freeze the memtable. This means that we do way more writeLock acquisitions than we need to...{quote}
> See CASSANDRA-1930 for backstory, but adding double checking inside a read lock before trying to re-entrantly acquire the writelock would eliminate most of these excess writelock acquisitions.
> Alternatively, we should explore removing locking from these structures entirely, and replacing the writeLock acquisition with a per-memtable counter of active threads.

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