cost_nonsequential_access()

Manfred Koizar <mkoi-pg@aon.at> writes:

The comment describing cost_nonsequential_access() says that the two
functions "meet in the middle". They meet at random_page_cost/2,
however, not in the middle between 1 and random_page_cost. For
random_page_cost < 2 the result can be less than 1 for relpages near
effective_cache_size. I don't think that this was intended.

You're right, I failed to consider that random_page_cost might be less
than 2.

This patch makes sure that cost_nonsequential_access() is always between
1 and randon_page_cost and the functions meet a (1+random_page_cost)/2.

This patch seems to do considerably more violence to the equations than
is needed to cover that oversight, though. The old behavior was
intentionally nonlinear in relsize; this is not.

regards, tom lane

On Tue, 08 Jun 2004 11:13:43 -0400, Tom Lane <tgl@sss.pgh.pa.us> wrote:

This patch seems to do considerably more violence to the equations than
is needed to cover that oversight, though. The old behavior was
intentionally nonlinear in relsize; this is not.

The comment says "entirely ad-hoc" and I didn't see a particular reason
why the lower half should be nonlinear in relsize while the upper half
is linear in 1/relsize. So I opted for the "more esthetic" symmetrical
function. :-)

http://www.pivot.at/pg/costsize.jpg original
http://www.pivot.at/pg/costsize_2.jpg nonlinear/linear
http://www.pivot.at/pg/costsize_3a.jpg nonlinear
http://www.pivot.at/pg/costsize_3b.jpg linear

I don't have a strong opinion for either relsize or relsize^2. So
please add " * relsize" or " / relsize" as appropriate before you apply
(if you intend to apply).

Servus
Manfred

Manfred Koizar <mkoi-pg@aon.at> writes:

On Tue, 08 Jun 2004 11:13:43 -0400, Tom Lane <tgl@sss.pgh.pa.us> wrote:

This patch seems to do considerably more violence to the equations than
is needed to cover that oversight, though. The old behavior was
intentionally nonlinear in relsize; this is not.

The comment says "entirely ad-hoc" and I didn't see a particular reason
why the lower half should be nonlinear in relsize while the upper half
is linear in 1/relsize.

Incremental changes in the relsize fraction are not going to change the
cost much except near 1, so I was after a curve that went like this
(pardon the crude artwork):

rpc ***********************
*******
****
**
cost *
*
*
**
****
********
1.0 *************
0 1 large
relsize fraction

I don't think replacing the lower half of this with a straight line
segment is an improvement.

Possibly the relsize axis ought to be measured on a log scale, or
something like that, but that didn't seem to work nicely when relsize
approaches zero.

regards, tom lane

On Tue, 08 Jun 2004 13:13:01 -0400, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Possibly the relsize axis ought to be measured on a log scale, or
something like that, but that didn't seem to work nicely when relsize
approaches zero.

In my experiments I used log(relsize) on the x axis, and I don't think
that the graph looks unpleasant for small relsize. My thought was (and
is) that we are much more interested in whether relpages is 1/100, 1/10,
1, 10, 100 times effective_cache_size than whether it is relpages +/-
1000, 2000, 3000, ...

I played around with some numbers that could be considered fairly
realistic. You might want to look at the graphs I linked to in the
previous message or download http://www.pivot.at/pg/costsize.sxc.

But I think we are wasting too much effort. The graphs don't look too
different, whether you use relsize or relsize^2. Maybe relsize^3 is
optimal? Nobody knows. The important part of the patch is that the
result is scaled and shifted into the range 1 to random_page_cost.
Whatever you decide to do is ok with me.

Servus
Manfred