[Dovecot] Possible sort optimization (?)

[Michael M Slusarz]

Quoting Timo Sirainen <tss at iki.fi>:

> On 6.2.2013, at 1.02, Michael M Slusarz <slusarz at curecanti.org> wrote:
>
>> a UID SORT RETURN (ALL COUNT) (DATE) UTF-8 SUBJECT "foo"
>>
>> is always about 10 percent slower than this split query (I've done  
>> this 4-5 times, and the numbers are similar):
>>
>> a UID SEARCH RETURN (SAVE) CHARSET UTF-8 SUBJECT "foo"
>> b UID SORT RETURN (ALL COUNT) (DATE) UTF-8 UID $
>>
>> (The particular query I used matched 5 messages out of the 21,000+)
>
> I think the main difference is that the first command fetches also  
> Date: header from dovecot.index.cache. Did you check if the slowness  
> was because of additional userspace CPU usage (instead of disk IO)?
>
> See if the attached patch makes a difference?

Without patch - single query (time output):

3.126u 2.763s 1:54.87 5.1%      0+0k 491192+19016io 11pf+0w
3.236u 2.663s 2:14.62 4.3%      0+0k 491064+18616io 9pf+0w

With patch - single query:

2.909u 2.689s 2:20.58 3.9%      0+0k 491064+15816io 11pf+0w
2.989u 2.673s 2:07.51 4.4%      0+0k 491056+15720io 11pf+0w

Userspace CPU usage dropped slightly, along with significant I/O.

(FYI: Split query with patch):
2.806u 2.696s 2:27.17 3.7%      0+0k 491200+11848io 11pf+0w
2.929u 2.626s 2:23.37 3.8%      0+0k 491064+11856io 11pf+0w

> One possible solution would be to do the prefetching when search  
> program is "all", but not otherwise. But if most of the messages  
> match the search query then this is slower..

Above results are from a search that matches 42 out of 26300 messages.

michael