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Re: Further reduction of bufmgr lock contention

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: josh(at)agliodbs(dot)com, pgsql-hackers(at)postgresql(dot)org, Gavin Hamill <gdh(at)acentral(dot)co(dot)uk>
Subject: Re: Further reduction of bufmgr lock contention
Date: 2006-05-30 01:29:08
Message-ID: 200605300129.k4U1T8022210@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Is this a TODO?

---------------------------------------------------------------------------

Tom Lane wrote:
> Josh Berkus <josh(at)agliodbs(dot)com> writes:
> > BTW, we're going to be testing this patch on Sun Niagara servers.   What's 
> > the outstanding bug with it?   I don't quite follow.
> 
> It's not acceptable as-is because of the risk of running out of shared
> memory for hashtable entries.  In the existing code, there's a clear
> upper bound on the number of entries in the block-number-to-buffer hash
> table, ie, shared_buffers + 1 (the +1 because we acquire the new entry
> before releasing the old when reassigning a buffer).  With multiple
> hashtables serving subsets of the buffers, the different tables might
> at different times need different numbers of entries, and that makes it
> a lot harder to be sure you won't run out of memory.  I don't say it's
> insoluble, but the current patch wasn't even claimed to be safe by its
> author...
> 
> 			regards, tom lane
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 5: don't forget to increase your free space map settings
> 

-- 
  Bruce Momjian   http://candle.pha.pa.us
  EnterpriseDB    http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +

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