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Date: Thu, 3 Sep 2009 15:44:57 -0700 (PDT)
From: "Chris Anderson (JIRA)" <jira@apache.org>
To: dev@couchdb.apache.org
Subject: [jira] Updated: (COUCHDB-495) Make views twice as fast
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     [ https://issues.apache.org/jira/browse/COUCHDB-495?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Chris Anderson updated COUCHDB-495:
-----------------------------------

    Attachment: term_collate.diff

This patch (term_collate.diff) gives an option to use raw Erlang term comparisons for the entire collation function. It is much faster than ICU collation (about 2x) and removes the single largest view generation bottleneck. It significantly changes view collation behavior (when the option is used) but that is to be expected.

However, it is not very clean, and it is incomplete. I've noted most of the shortcomings in the ticket description so I won't restate them.

If you plan to work on the bug, I'd suggest you NOT start by applying this patch, but rather, read and understand it, so you can see why I suggest the refactorings listed in the ticket. This patch is basically a dead end, but a good learning experience...

> Make views twice as fast
> ------------------------
>
>                 Key: COUCHDB-495
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-495
>             Project: CouchDB
>          Issue Type: Improvement
>          Components: JavaScript View Server
>            Reporter: Chris Anderson
>             Fix For: 0.11
>
>         Attachments: binary_collate.diff, term_collate.diff
>
>
> Devs,
> Damien's identified view collation as the most significant bottleneck for the view generation. We've done some testing, and some preliminary patches, and the upshot seems to be that even removing ICU from the collator is not a significant boost. What does speed things up greatly is using raw Erlang term comparison. Eg, instead of using couch_view:less_json, using fun(A,B) A < B end. provides a roughly 2x speedup.
> However, the patch is challenging for a few reasons: Making the collation strategy switchable at all is tough. It's actually quite easy to get an alternate less function into the btree writer (all you've got to do is set it in couch_view_group:init_group). The hard part is propagating the same less function to the PassedEndFun. There's a secondary problem that when you use raw term comparison, a lot of terms turn out to come before nil, and after {}, which we use as artificial first and last terms in the less_json function. So just switching to raw collation alone will leave you with a view with unreachable rows.
> I tried two different approaches to the problem last night, and both of them led to (instructive) dead ends. I'll attach them for illustration purposes.
> The next line of attack we think should be tried is this:
> First - remove _all_docs_by_seq, as it is just adding complexity to the problem, and has been deprecated by _changes anyway. Along the same lines, _all_docs should no longer use couch_httpd_view:make_view_fold_fun as it has completely different collation needs than make_view_fold_fun. We'll end up duplicating a little code in the _all_docs implementation, but it should be worth it because it will make the other work much simpler.
> Once those changes have laid the groundwork, the next step is to change make_view_fold_fun and couch_view:fold, so that rather than make_view_fold_fun being responsible for detecting when we've passed the endkey. That means make_passed_end_fun and all references to PassedEnd and PassedEnd fun will be stripped from couch_httpd_view and moved to couch_btree.
> couch_view:fold (and the underlying btree) will need to accept not just a start, but also an endkey. This will make it much easier to use the less fun that is stored on View#view.btree#btree.less to determine PassedEnd funs. This will move some complexity to the btree code from the view code, but will keep the concerns more aligned. This also means that the btree will need to accept not only an endkey for folds, but also an inclusive_end parameter.
> Once we have all these refactorings done, it will be easy to make the less fun for an index configurable, as both the index writer and the index reader will look for it in the same place (on the #btree record).
> My aim is to start a discussion and get someone excited to work on this patch. Think of all the fast-views glory you'll get! Please ask questions and otherwise force me to clarify the above discussion.

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