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From "Jay Kreps (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (KAFKA-191) Investigate removing the synchronization in Log.flush
Date Sun, 06 Nov 2011 03:22:54 GMT

    [ https://issues.apache.org/jira/browse/KAFKA-191?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13144898#comment-13144898
] 

Jay Kreps commented on KAFKA-191:
---------------------------------

Neha--Sounds good. I have a patch (since it is just deleting and reordering, the code change
itself is trivial), I will attach. Here are my thoughts. I think we can just remove the synchronization
and re-order things so that the unflushed counter and lastFlushTime both remain valid lower
bounds. It is possible that the time we set could get overwritten by another thread but it
is unlikely to make any practical difference. See if you agree with that logic, I am not 100%
positive.

I am not at all sure that this will actually help performance though for two reasons. First,
I think it is possible that the file itself may be synchronized. Either at the java level
or the OS level. So I am not sure if one can write to the file while a flush is occurring
in another thread. This may take some research to understand. 

Second, if it is possible to do parallel write and flush, I think this still may not be ideal
(though maybe a good short term hack). My reasoning is that this patch only fixes the blocking
behavior for the time-based flush, but my question is why would I ever want to block?

I really see two use cases:
1. I want every message I write immediately flushed to disk in a blocking fashion before the
append() is considered completed. This corresponds to flush.interval=1 in the current system.
2. I want to periodically flush data, which could be based on the number of messages, or time,
(or theoretically based on unflushed bytes, though we haven't implemented that).

So what I am thinking is that case (1) clearly needs to be blocking to make sense. But for
any periodic flush I don't see a reason to block appends. It is true that this makes the intervals
inexact, but I think that is probably fine.. For example, even if I set flush.interval=5,
it is unlikely that I could actually care that it is exactly 5, I just want to flush often,
say ~5 messages. (Even if I did want it exact, since we always write the full messageset,
I still might not get that). So I am thinking a better long-term approach might be to have
a central threadpool that handles all flushing and have that always be asynchronous. So if
I set flush.interval=5, then that means the background thread is triggered every 5 messages
BUT no one blocks on this. In addition to this we add an immediate.commit=true/false option
to force data to be flushed in a blocking way as part of the append.

Obviously the above only works if a parallel append and flush are possible.
                
> Investigate removing the synchronization in Log.flush
> -----------------------------------------------------
>
>                 Key: KAFKA-191
>                 URL: https://issues.apache.org/jira/browse/KAFKA-191
>             Project: Kafka
>          Issue Type: New Feature
>          Components: core
>    Affects Versions: 0.8
>            Reporter: Jay Kreps
>            Assignee: Neha Narkhede
>         Attachments: KAFKA-191.patch
>
>
> Currently we have the following synchronization in Log.scala:
> 1. append, roll, and flush all share a write lock
> 2. read is non-blocking
> Unfortunately this means that flush time latency is added to appends (even if the flush
is done by a background thread). To fix this we should investigate a scheme to make append
and flush not block each other.

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