Date: Tue, 5 Mar 2002 18:10:13 -0500 (EST) From: Zhihui Zhang <zzhang@cs.binghamton.edu> To: Lars Eggert <larse@ISI.EDU> Cc: "Rogier R. Mulhuijzen" <drwilco@drwilco.net>, Julian Elischer <julian@elischer.org>, freebsd-hackers@FreeBSD.ORG Subject: Re: A weird disk behaviour Message-ID: <Pine.SOL.4.21.0203051803150.13181-100000@onyx> In-Reply-To: <3C854EA4.5040306@isi.edu>
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Well, the core of my program is as follows (RANDOM(x) return a value
between 0 and x):
blocksize = 8192;
write_size_low = 512;
time(&time1);
for (i = 0; i < write_count; i++) {
write_size = write_size_low +
RANDOM(write_size_high-write_size_low);
write_size = roundup(write_size, DEV_BSIZE);
if (testcase == 1)
write_size = blocksize;
write_block(rawfd, sectorno, buf, write_size);
sectorno += blocksize / DEV_BSIZE;
}
time(&time2);
If testcase is one, then the time elapsed (time2 - time1) is much less.
-Zhihui
On Tue, 5 Mar 2002, Lars Eggert wrote:
> I agree that it's probably caching at some level. You're only writing
> about 120MB of data (and half that in your second case). Bump these to a
> couple of GB and see what happens.
>
> Also, could you post your actual measurements?
>
> Lars
>
>
> Zhihui Zhang wrote:
> > The machine has 128M memory. I am doing physical I/O one block at a time,
> > so there should be no memory copy.
> >
> > -Zhihui
> >
> > On Tue, 5 Mar 2002, Rogier R. Mulhuijzen wrote:
> >
> >
> >>At 16:03 5-3-2002 -0500, Zhihui Zhang wrote:
> >>
> >>
> >>
> >>>On Tue, 5 Mar 2002, Julian Elischer wrote:
> >>>
> >>>
> >>>>more writes fit in the disk's write cache?
> >>>>
> >>>For (1), it writes 15000 * 8192 bytes in all. For (2), it writes 15000 *
> >>>4096 bytes in all (assuming the random number distributes evenly between 0
> >>>and 8192). So your suggestion does not make sense to me.
> >>>
> >>How large is your buffercache? it might be that the 15000 * ~4096 roughly
> >>matches with your cache, and 15000 * 8912 doesn't.
> >>
> >>Case (1) would require a lot more physical IO in that case than case (2)
> >>would require.
> >>
> >> Doc
> >>
> >>
> >>
> >>>-Zhihui
> >>>
> >>>
> >>>>On Tue, 5 Mar 2002, Zhihui Zhang wrote:
> >>>>
> >>>>
> >>>>>I am doing some raw I/O test on a seagate SCSI disk running FreeBSD 4.5.
> >>>>>This situation is like this:
> >>>>>
> >>>>> +-----+----+----+----+----+----+----+----+----+----+---+------
> >>>>> | | | | | | | | | | | | ....
> >>>>> +-----+----+----+----+----+----+----+----+----+----+---+------
> >>>>>
> >>>>>Each block is of fixed size, say 8192 bytes. Now I have a user program
> >>>>>writing each contiguously laid out block sequentially using /dev/daxxx
> >>>>>interface. There are a lot of them, say 15000. I write the blocks in two
> >>>>>ways (the data used in writing are garbage):
> >>>>>
> >>>>>(1) Write each block fully and sequentially, ie. 8192 bytes.
> >>>>>
> >>>>>(2) I still write these blocks sequentially, but for each block I only
> >>>>>write part of it. Exactly how many bytes are written inside each
> >>>>>
> >>>block is
> >>>
> >>>>>determinted by a random number between 512 .. 8192 bytes (rounded up a
> >>>>>to multiple of 512 bytes).
> >>>>>
> >>>>>I find out the the performance of (2) is several times better than the
> >>>>>performance of (1). Can anyone explain to me why this is the case?
> >>>>>
> >>>>>Thanks for any suggestions or hints.
> >>>>>
> >>>>>-Zhihui
> >>>>>
> >>>>>
> >>>>>
> >>>>>To Unsubscribe: send mail to majordomo@FreeBSD.org
> >>>>>with "unsubscribe freebsd-hackers" in the body of the message
> >>>>>
> >>>>>
> >>>>
> >>>
> >>>To Unsubscribe: send mail to majordomo@FreeBSD.org
> >>>with "unsubscribe freebsd-hackers" in the body of the message
> >>>
> >>
> >
> >
> > To Unsubscribe: send mail to majordomo@FreeBSD.org
> > with "unsubscribe freebsd-hackers" in the body of the message
> >
>
>
>
> --
> Lars Eggert <larse@isi.edu> Information Sciences Institute
> http://www.isi.edu/larse/ University of Southern California
>
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