Re: [HACKERS] Sequential scan speed, mmap, disk i/o

> > Someone was complaining about sequential scan speed, so I decided to run
> > a test.
>
> > wc 41 sec
> > wc -l 31 sec
> > dd if=/u/pg/data/base/test/testv of=/dev/null bs=512 32 sec
> > dd if=/u/pg/data/base/test/testv of=/dev/null bs=8k 31 sec
> > dd if=/u/pg/data/base/test/testv of=/dev/null bs=256k 31 sec
> > dd if=/u/pg/data/base/test/testv of=/dev/null bs=1m 30 sec
> > mmap() of file in 8k chunks 99 sec
> > mmap() of file in 8mb chunks 40 sec
> > mmap() of file in 32mb chunks 56 sec
> >
> > PostgreSQL sequential scan 37 sec
>
> Let me add, these times are on a PP200, with SCSI Ultra Barracuda
> drives, BSD/OS 3.1, 64MB RAM.
>
> --
> Bruce Momjian | 830 Blythe Avenue
> maillist(at)candle(dot)pha(dot)pa(dot)us | Drexel Hill, Pennsylvania 19026
> + If your life is a hard drive, | (610) 353-9879(w)
> + Christ can be your backup. | (610) 853-3000(h)

Very interesting. Is it possible to get the schema, the query, and a
a sample of the data or a generator program for the data? I am quite surprised
to see us do so well, I would have guess that the per row overhead would
have us down far below wc.

Although, on second though, dd has to write the data as well as read it, and
we don't, and wc has to examine every character, where if the "where clause"
applies to only a portion of the row, we don't.

Still, It would be nice to see more info about this test.

Btw, I hope to post tommorrow some hopefully interesting results about the
speed of TAS and S_LOCK...

-dg

David Gould dg(at)illustra(dot)com 510.628.3783 or 510.305.9468
Informix Software (No, really) 300 Lakeside Drive Oakland, CA 94612
"Of course, someone who knows more about this will correct me if I'm wrong,
and someone who knows less will correct me if I'm right."
--David Palmer (palmer(at)tybalt(dot)caltech(dot)edu)