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Re: bash history with mixed timestamps slow and broken


From: Chet Ramey
Subject: Re: bash history with mixed timestamps slow and broken
Date: Wed, 5 Oct 2016 09:18:43 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:45.0) Gecko/20100101 Thunderbird/45.3.0

On 9/25/16 6:39 PM, Chet Ramey wrote:
> On 9/24/16 2:17 PM, Hubert Schmid wrote:

>> Bash Version: 4.4
>> Patch Level: 0
>> Release Status: release
>>
>> Description:
>>      If the history file (`.bash_history`) starts with a timestamp
>>      (`HIST_TIMESTAMP_START`), and contains lines that have been written
>>      without timestamps, then reading the history file is (a) very slow
>>      because (b) consecutive lines without timestamps are merged into a
>>      single history entry with quadratic complexity.
>>
>>      Apparently, this problem didn't exist in the previous version (4.3).
> 
> One of the most frequently-requested features for the bash and readline
> history implementations is a way to preserve multi-line commands across
> shell sessions.  It's been the subject of numerous previous discussions
> on bug-bash.
> 
> There hasn't been a good way to do that, since there isn't good support
> for it in the traditional readline (flat) history file format.
> 
> I decided to implement a heuristic: if the history file starts with a
> timestamp, and the application using the history library -- in this case,
> bash -- has indicated that it's interested in writing timestamps to the
> history file (which is off by default), the history file reading code
> assumes that timestamps appear consistently in the history file and it can
> use them as markers to delimit commands.
> 
> The appending behavior isn't really quadratic: the code simply keeps
> reallocating the line and appending to it.  You can imagine how long it
> takes to append a million commands to a single buffer.  You've managed
> to identify the most degenerate case.

Try the attached patch and see whether or not it improves the load time.
You're still going to end up with one very long history entry.

Chet
-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/

Attachment: history-appendline.patch
Description: Text Data


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