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From Henrik Thostrup Jensen <thost...@gmail.com>
Subject Re: Performance Regression for view generation in 0.11
Date Mon, 15 Mar 2010 10:10:47 GMT
Hi

On 13 March 2010 20:44, Benoit Chesneau <bchesneau@gmail.com> wrote:

> delayed_commits = true ; set this to false to ensure an fsync before
> 201 Created is returned
> batch_save_size = 1000 ; number of docs at which to save a batch
> batch_save_interval = 1000 ; milliseconds after which to save batches
>
> are the settings you may want to change in ini.

This appears to work in 0.10, but is (as far as I can tell) ignored in
0.11, due to the changed view collection architecture, which relies on
work queues, which have statically defined sizes (see line 42-43 in
couch_view_updater.erl).

I have a synthetic benchmark for view generation over 70K documents.
In stock CouchDB 0.10, the view will be checkpointed about 15-17
times. Around 9 times with the batch_save_size and batch_save_interval
set to 10000. CouchDB 0.11 on the other hand performs a whopping
108/109 checkpoints of the view. Due to shadow B-trees this generates
significantly larger view files (2-3x much) and more time is spend
writing to disk. Generating the view takes roughly twice as long in
0.11 as it does in 0.10.

I've tracked down the problem to the new view generation architecture;
particularly the small sizes of the work queues defined in
couch_view_updater.erl. The attached patch decreases the number of
checkpoints to around 15, and makes view generation slightly faster
than 0.10. It basically increases the size of the write queue.
Inserting a 500 ms sleep in do_writes increased the performance a bit
more, but that is not a nice or right solution.

I suspect the patch is not "the completely right solution (tm)", as a
lot checkpoints are performed initially whereafter it backs off and
starts to take longer time/revisions between the checkpoints. I
suspect that the code is just writing repeatedly and as writes start
to take longer time, more revisions are added per checkpoint. Though I
am not really sure of this.

Still, it is a 2 line patch, and it significantly increases view
generation performance. I'd very much like to see this in 0.11, to
avoid a rather large performance regression between 0.10 and 0.11. If
0.11 comes out as it is, we would either have to stick with 0.10 or
build our own patched 0.11.

-- 
   - Henrik

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