hbase-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Ryan Rawson <ryano...@gmail.com>
Subject Re: Good VLDB paper on WALs
Date Wed, 29 Dec 2010 20:07:52 GMT
Oh no, let's be wary of those server rewrites.  My micro profiling is
showing about 30 usec for a lock handoff in the HBase client...

I think we should be able to get big wins with minimal things.  A big
rewrite has it's major costs, not to mention to effectively be async
we'd have to rewrite every single pice of code more complex than
Bytes.*.  If you need to block you will need to push context on a
context-store (aka stack) and manage that all ourselves.

I've been seeing papers that are talking about threading improvements
that could get us better performance.  Assuming that ctx is the actual
reason why we arent as fast as we could be (note: we are NOT slow!).

As for the DI, I think I'd like to see more study on the costs and
benefits.  We have a relatively minimal amount of interfaces and
concrete objects, for the interfaces we do, we have 1 or 2
implementations at most.  Usually 1.  There is a cost, I'd like to see
more descriptions of the costs vs the benefits.


On Wed, Dec 29, 2010 at 11:32 AM, Stack <stack@duboce.net> wrote:
> Nice list of things we need to do to make logging faster (with useful
> citations on current state of art).  This notion of early lock release
> (ELR) is worth looking into (Jon, for high rates of counter
> transactions, you've been talking about aggregating counts in front of
> the WAL lock... maybe an ELR and then a hold on the transaction until
> confirmation of flush would be way to go?).  Regards flush-pipelining,
> it would be interesting to see if there are traces of the sys-time
> that Dhruba is seeing in his NN out in HBase servers.  My guess is
> that its probably drowned by other context switches done in our
> servers.  Definitely worth study.
> St.Ack
> P.S. Minimizing context switches, a system for ELR and
> flush-pipelining, recasting the server to make use of one of the DI or
> OSGi frameworks, moving off log4j, etc..... Is it just me or do others
> feel a server rewrite coming on?
> On Mon, Dec 27, 2010 at 11:48 AM, Dhruba Borthakur <dhruba@gmail.com> wrote:
>> HDFS currently uses Hadoop RPC and the server thread blocks till the WAL is
>> written to disk. In earlier deployments, I thought we could safely ignore
>> flush-pipelining by creating more server threads. But in our largest HDFS
>> systems, I am starting to see  20% sys-time usage on the namenode machine;
>> most of this  could be thread scheduling. If so, then it makes sense to
>> enhance the logging code to release server threads even before the WAL is
>> flushed to disk (but, of course, we still have to delay the transaction
>> response to the client till the WAL is synced to disk).
>> Does anybody have any idea on how to figure out what percentage of the above
>> sys-time is spent in thread scheduling vs the time spent in other system
>> calls (especially in the Namenode context)?
>> thanks,
>> dhruba
>> On Fri, Dec 24, 2010 at 8:17 PM, Todd Lipcon <todd@cloudera.com> wrote:
>>> Via Hammer - I thought this was a pretty good read, some good ideas for
>>> optimizations for our WAL.
>>> http://infoscience.epfl.ch/record/149436/files/vldb10aether.pdf
>>> -Todd
>>> --
>>> Todd Lipcon
>>> Software Engineer, Cloudera
>> --
>> Connect to me at http://www.facebook.com/dhruba

View raw message