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From "Josh Elser" <josh.el...@gmail.com>
Subject Re: Review Request 28873: ACCUMULO-3393 Follow-on work for per-table volume chooser.
Date Wed, 10 Dec 2014 17:16:08 GMT


> On Dec. 10, 2014, 7:54 a.m., Christopher Tubbs wrote:
> > server/base/src/main/java/org/apache/accumulo/server/fs/PreferredVolumeChooser.java,
lines 125-141
> > <https://reviews.apache.org/r/28873/diff/1/?file=787889#file787889line125>
> >
> >     How is this implementation better than the simple, readable, higher-level collections-based
implementation?
> 
> Sean Busbey wrote:
>     VC.choose is in the critical path for tablet splitting. The collections based approach
does more allocations of much heavier weight objects. Additionally, since all of these things
only live in the scope of the method those allocations will cause gc pressure during heavy
splitting.
> 
> Josh Elser wrote:
>     bq. will cause gc pressure during heavy splitting.
>     
>     *might* cause gc pressure :). I don't think you can make that assertion without actually
quantifying how much "gc pressure" is introduced WRT the amount of time the other known slow
operations with comprise splitting a tablet actually take.
> 
> Sean Busbey wrote:
>     Am I correct that we currently don't have any microbenchmarks that would tell us
one way or the other?

I imagine that you could easily look at how long it takes a split to happen with and without
ingest load via the traces. If we don't have that instrumented already, maybe Eric or Keith
could chime in as I imagine they would be the two who know the most off the top of their heads.
I don't want to force you to start writing microbenchmarks -- my gut reaction was that back
of the envelope calculations would hint that this optimization might not be with much total
improvement.


> On Dec. 10, 2014, 7:54 a.m., Christopher Tubbs wrote:
> > server/base/src/main/java/org/apache/accumulo/server/fs/PreferredVolumeChooser.java,
line 99
> > <https://reviews.apache.org/r/28873/diff/1/?file=787889#file787889line99>
> >
> >     Do we really need an object pool for a map which holds 0 or 1 element for the
duration of a single, infrequently called method? A better optimization for this case would
be to add an optimized get(String key) method to AccumuloConfiguration / TableConfiguration,
so we don't have to use a map at all.
> 
> Sean Busbey wrote:
>     This code path is in the critical section of tablet splitting, the frequency of which
varies greatly based on system deployment. There's already a TODO for doing the refactoring
you mention, which will allow us to later remove the pooling entirely.
> 
> Josh Elser wrote:
>     I really don't see the creation of a new HashMap as being critical WRT the cost of
waiting for a tablet to close, performing multiple metadata table updates and then waiting
for the tablets to come back online.
> 
> Sean Busbey wrote:
>     Am I correct that we currently don't have any microbenchmarks that would tell us
one way or the other?

ditto what I said down below.


> On Dec. 10, 2014, 7:54 a.m., Christopher Tubbs wrote:
> > server/base/src/main/java/org/apache/accumulo/server/fs/PreferredVolumeChooser.java,
line 44
> > <https://reviews.apache.org/r/28873/diff/1/?file=787889#file787889line44>
> >
> >     This is incorrect. This is not an experimental property. It is a custom per-table
property, specific to the PreferredVolumeChooser.
> 
> Sean Busbey wrote:
>     how would you prefer I convey the information that operators can't assume any compatibility
guarantees on its use?
> 
> Josh Elser wrote:
>     I don't get this either. The property is defined to take a comma-separated list of
Volumes (URIs). If this fact changes, we'd have to make a new property to support whatever
we needed it to become. Experimental has been used to define things that are incompletely
implemented -- anything else has been handled by documentation.
> 
> Sean Busbey wrote:
>     I'm trying to convey a similar level of reliability as we do with @Experimental on
GENERAL_VOLUME_CHOOSER and TABLE_VOLUME_CHOOSER. That is, we might change the variable name
or what it holds. Would describing it as a "table property specific to the current implementation
of PreferredVolumeChooser" work?

Hrm, I hadn't noticed that GENERAL_VOLUME_CHOOSER was also Experimental so that makes sense
for it to also trickle down to TABLE_VOLUME_CHOOSER. I don't think I mind experimental moving
down the chain to keep all of these related. The PREFERRED_VOLUMES_CUSTOM_KEY by itself may
not be experimental (by the previous definition), but since it relies on other things which
are still marked as such, I could see the reason behind also treating it as experimental.


- Josh


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https://reviews.apache.org/r/28873/#review64511
-----------------------------------------------------------


On Dec. 10, 2014, 2:58 p.m., Sean Busbey wrote:
> 
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://reviews.apache.org/r/28873/
> -----------------------------------------------------------
> 
> (Updated Dec. 10, 2014, 2:58 p.m.)
> 
> 
> Review request for accumulo and Jenna Huston.
> 
> 
> Bugs: ACCUMULO-3393
>     https://issues.apache.org/jira/browse/ACCUMULO-3393
> 
> 
> Repository: accumulo
> 
> 
> Description
> -------
> 
> ACCUMULO-3393 Follow-on work for per-table volume chooser.
> 
> Still has TODOs for additional tickets I need to file. I'll update the review to remove
once I have them all filed. I think everything marked TODO can wait for a later ticket. Please
comment on relevant section if you think something needs to be done now.
> 
> * docs clean up + code guideline compliance.
> * ensure RandomVolumeChoosers are independent when used per-table.
> * make sure that per-table choosers can keep state the way that global choosers can
> * make sure that a chooser can only pick from the options it is presented.
> * avoid object creation in critical path for Tablet.split.
> 
> 
> Diffs
> -----
> 
>   core/src/main/java/org/apache/accumulo/core/conf/Property.java 4c2d0b41b3bce32449861da3ac42fa27deb2b182

>   pom.xml 31601a1bf84e19b861e4f48b50824eeb77987b52 
>   server/base/pom.xml c21a168dec2092692f1ee6877c4703ee2d3e3977 
>   server/base/src/main/java/org/apache/accumulo/server/fs/PerTableVolumeChooser.java
7a825c796eb5a25de8c748e2aba642f483697b9a 
>   server/base/src/main/java/org/apache/accumulo/server/fs/PreferredVolumeChooser.java
7ed7bba809a4e5e4b2d472c3327b15adb37251a7 
>   server/base/src/main/java/org/apache/accumulo/server/fs/RandomVolumeChooser.java f2eb2113cb848ed58ac5f41573c6ff2cde9b0a77

>   server/base/src/main/java/org/apache/accumulo/server/fs/VolumeChooser.java f523057b11a2dc42e82010675bb1ac8e3802f96d

>   server/base/src/main/java/org/apache/accumulo/server/fs/VolumeManager.java 890651e92f4c34514cb839b7b9ee9d23ad55070a

>   server/base/src/main/java/org/apache/accumulo/server/fs/VolumeManagerImpl.java dc1be739b634d91992894f8f27c2d9c184bd98cd

>   server/base/src/main/java/org/apache/accumulo/server/fs/VolumeUtil.java 877d01c2233cca086c1ac1539eb81cc282a7ceb4

>   server/base/src/main/java/org/apache/accumulo/server/util/TabletOperations.java c0e1a9b991d61a4dbb127c74ae297f171434e7d5

>   server/base/src/test/java/org/apache/accumulo/server/fs/VolumeManagerImplTest.java
582822a8ccbc398925a2184eed0b9d7fa853f9b4 
> 
> Diff: https://reviews.apache.org/r/28873/diff/
> 
> 
> Testing
> -------
> 
> Ran through VolumeManagerImplTest and VolumeChooserIT 
> 
> 
> Thanks,
> 
> Sean Busbey
> 
>


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