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Re: SSI patch version 14

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Dan Ports <drkp(at)csail(dot)mit(dot)edu>
Cc: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, simon(at)2ndquadrant(dot)com, markus(at)bluegap(dot)ch, pgsql-hackers(at)postgresql(dot)org
Subject: Re: SSI patch version 14
Date: 2011-02-09 02:09:48
Message-ID: AANLkTikc8rHjfm_JCSZtwqEHYFO_zr0i=bgP-hiBSWkF@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, Feb 8, 2011 at 7:23 PM, Dan Ports <drkp(at)csail(dot)mit(dot)edu> wrote:
> On Tue, Feb 08, 2011 at 04:04:39PM -0600, Kevin Grittner wrote:
>> (2)  The predicate lock and lock target initialization code was
>> initially copied and modified from the code for heavyweight locks.
>> The heavyweight lock code adds 10% to the calculated maximum size.
>> So I wound up doing that for PredicateLockTargetHash and
>> PredicateLockHash, but didn't do it for SerializableXidHassh.
>> Should I eliminate this from the first two, add it to the third, or
>> leave it alone?
>
> Actually, I think for SerializableXidHash we should probably just
> initially allocate it at its maximum size. Then it'll match the
> PredXact list which is allocated in full upfront, and there's no risk
> of being able to allocate a transaction but not register its xid. In
> fact, I believe there would be no way for starting a new serializable
> transaction to fail.

No way to fail is a tall order.

If we don't allocate all the memory up front, does that allow memory
to be dynamically shared between different hash tables in shared
memory?  I'm thinking not, but...

Frankly, I think this is an example of how our current shared memory
model is a piece of garbage.  Our insistence on using sysv shm, and
only sysv shm, is making it impossible for us to do things that other
products can do easily.  My first reaction to this whole discussion
was "who gives a crap about 2MB of shared memory?" and then I said
"oh, right, we do, because it might cause someone who was going to get
24MB of shared buffers to get 16MB instead, and then performance will
suck even worse than it does already".  But of course the person
should really be running with 256MB or more, in all likelihood, and
would be happy to have us do that right out of the box if it didn't
require them to do tap-dance around their kernel settings and reboot.

We really need to fix this.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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