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Re: Worried about single-db performance

From: Matthew Bentham <mjb67_at_artvps.com>
Date: Tue, 07 Sep 2010 13:29:15 +0100

On 07/09/2010 13:02, Bert Huijben wrote:
>
>
>> -----Original Message-----
>> From: Matthew Bentham [mailto:mjb67_at_artvps.com]
>> Sent: dinsdag 7 september 2010 13:48
>> To: Bert Huijben
>> Cc: 'Justin Erenkrantz'; 'Greg Stein'; 'Johan Corveleyn'; 'Subversion
>> Development'
>> Subject: Re: Worried about single-db performance
>
>
>> I didn't realise this, you are of course right that that would make it
>> unacceptable. I don't really understand why it would break
>> TortoiseSVN,
>> does it take write access and then not release it somehow?
>
> SQLite needs a shared *read* lock to *read*. See
> http://www.sqlite.org/atomiccommit.html.
> (Invoking 'svn status' never obtains a write lock; see that document)
>
> SQLite only upgrades that read lock to a write (or actually reserved) lock
> when you perform a db operation that has to change the database. Further on
> (E.g. too many changes, but look at the documentation for more reasons) this
> is upgraded to an exclusive lock that blocks all readers and writers out of
> the db, but it tries to keep this time as short as possible.
>
> Your original suggestion is just to make any *reader* block any other
> *reader*. Which breaks the subversion world. (Just running svn update in 1.6
> has about 5 simultaneous independent readers in some phases of update. Most
> GUI subversion clients I know use multiple client instances at the same
> time, so they would all have to be rewritten if we obtain an exclusive lock
> for reading).

Sorry, I didn't mean we should take exclusive locks for every
transaction, just that we should use "PRAGMA locking_mode=EXCLUSIVE".
According to the documentation (http://www.sqlite.org/pragma.html) that
makes transactions obtain a shared lock for reading which is upgraded to
an exclusive lock for writing, and not released until the database
connection is closed. I've tried it a couple of times in svn.exe and it
always improves performance (over locking_mode=NORMAL) and hasn't caused
me problems. Admittedly I haven't tried within the last couple of weeks
and I'm afraid I don't have time right now.

Am I misreading the documentation? It says "The first time the database
is read in EXCLUSIVE mode, a shared lock is obtained and held. The first
time the database is written, an exclusive lock is obtained and held".

Matthew
Received on 2010-09-07 14:30:09 CEST

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