Re: data on devel code perf dip

On Thu, Aug 11, 2005 at 09:02:03PM -0400, Tom Lane wrote:

> I am sure I will get some pushback if I propose reverting the O_DIRECT
> patch, so could you try to get some more-specific evidence? Like pull
> the CVS tree from just before and just after this patch and compare
> performance?

Quoth the open(2) manpage:

O_DIRECT
Try to minimize cache effects of the I/O to and from this
file. In general this will degrade performance, but it is
useful in special situations, such as when applications do
their own caching. File I/O is done directly to/from user
space buffers. The I/O is synchronous, i.e., at the comple-
tion of the read(2) or write(2) system call, data is guaran-
teed to have been transferred.

In light of this, may I ask whether it makes sense to compare the
performance of two runs with similar shared_buffer settings? With
O_DIRECT, I understand from this manpage that the OS is going to do
little or no page caching, so shared_buffers should be increased to
account for this fact.

Am I missing something?

--
Alvaro Herrera (<alvherre[a]alvh.no-ip.org>)
"Hay que recordar que la existencia en el cosmos, y particularmente la
elaboración de civilizaciones dentre de él no son, por desgracia,
nada idílicas" (Ijon Tichy)