Date: Tue, 5 Mar 2002 18:19:31 -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.4.21.0203051818510.13181-100000@onyx> In-Reply-To: <3C855278.9040504@isi.edu>
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Several times slower! The point is that writing less data performs
worse. So I call it weird.
-Zhihui
On Tue, 5 Mar 2002, Lars Eggert wrote:
> Zhihui Zhang wrote:
> > 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.
>
> How "much less" in milliseconds?
>
> Also, in your original mail, you said you had 15,000 of these 8K blocks,
> which is only 120MB or so. Use 150,000 or 1,500,000 and check your
> results then.
>
> Lars
>
>
>
> > -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
> >>
> >>
> >
>
>
>
> --
> Lars Eggert <larse@isi.edu> Information Sciences Institute
> http://www.isi.edu/larse/ University of Southern California
>
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