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https://issues.apache.org/jira/browse/COUCHDB-1130?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13021318#comment-13021318
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Randall Leeds commented on COUCHDB-1130:
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@Paul: I have no idea.
And looking at the docs I think I understand a bit better now.
I think what we're doing is akin to the second code example in
http://www.erlang.org/doc/efficiency_guide/binaryhandling.html#id64095
Except, according to that excerpt, even if we switched the argument order (as
in my patch), we avoid creating sub-binaries but we still create a extra match
contexts.
This code path's only ever hit when we're pulling objects > 4096KB
(?SIZE_BLOCK) out of the couch file, and even then I expect the gains are quite
small. But who knows. I'll try to profile it in a bit. I'm super curious and
I'm enjoying the education.
> binary optimization in couch_file
> ---------------------------------
>
> Key: COUCHDB-1130
> URL: https://issues.apache.org/jira/browse/COUCHDB-1130
> Project: CouchDB
> Issue Type: Improvement
> Components: Database Core
> Affects Versions: 1.0.2
> Reporter: Randall Leeds
> Priority: Minor
> Attachments:
> 0001-refactor-remove_block_prefixes-2-for-optimization.patch
>
>
> I've had this patch sitting around since January and kept forgetting to file
> the ticket. Hurray spring cleaning.
> Just for fun I ran erlc with +bin_opt_info, which gives information about how
> the Erlang VM can optimize creating binary objects.
> What follows is the commit message from my patch.
> Even if I'm wrong about the last point, it can't hurt.
> What think you all?
> -------------------
> Running erlc with +bin_opt_info gives an INFO message stating that "matching
> anything else but a plain variable to the left of a binary pattern will
> prevent
> delayed sub binary optimization; SUGGEST changing argument order"
> I guess matching 0 is triggering this. If I understand correctly, this change
> will allow the compiler to skip creating a sub-binary that starts at the block
> boundary in the third clause and delay creation until we strip the leading
> byte
> in the 0 clause. This means one less 1-byte binary every time we read across a
> block boundary.
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