To: pgsql-performance@postgresql.org
From: Ivan Voras <ivoras@freebsd.org>
Subject: Re: [HACKERS] MIT benchmarks pgsql multicore (up to 48)performance
Date: Thu, 07 Oct 2010 00:31:19 +0200
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On 10/04/10 20:49, Josh Berkus wrote:

>> The other major bottleneck they ran into was a kernel one: reading from
>> the heap file requires a couple lseek operations, and Linux acquires a
>> mutex on the inode to do that. The proper place to fix this is
>> certainly in the kernel but it may be possible to work around in
>> Postgres.
> 
> Or we could complain to Kernel.org.  They've been fairly responsive in
> the past.  Too bad this didn't get posted earlier; I just got back from
> LinuxCon.
> 
> So you know someone who can speak technically to this issue? I can put
> them in touch with the Linux geeks in charge of that part of the kernel
> code.

Hmmm... lseek? As in "lseek() then read() or write()" idiom? It AFAIK
cannot be fixed since you're modifying the global "strean position"
variable and something has got to lock that.

OTOH, pread() / pwrite() don't have to do that.