Date: Sun, 19 Sep 1999 23:11:48 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Greg Lehey <grog@lemis.com> Cc: Poul-Henning Kamp <phk@critter.freebsd.dk>, dg@root.com, cvs-committers@FreeBSD.ORG, cvs-all@FreeBSD.ORG Subject: Re: User block device access (was: cvs commit: src/sys/miscfs/specfs spec_vnops.c src/sys/sys vnode.h src/sys/kern vfs_subr.c) Message-ID: <199909200611.XAA77678@apollo.backplane.com> References: <16748.937762251@critter.freebsd.dk> <199909191806.LAA73255@apollo.backplane.com> <19990920115556.J55065@freebie.lemis.com> <199909200527.WAA77336@apollo.backplane.com> <19990920152437.W55065@freebie.lemis.com>
next in thread | previous in thread | raw e-mail | index | archive | help
:That has nothing to do with the effect I noted, which may be a red
:herring.
:
:I've just checked, one reason for the slower transfer is because the
:dd transfer from the raw device always reads 2 kB per transfer, no
:matter what you tell dd. Here's iostat output for the following
:commands:
When running from the block device it is using the block size from
the label. For the 'c' partition this is normally 0, resulting in
the system picking '2K'.
Try running it from a partition other then c and set the blocksize
in the partition (you may have to set the fstype to 4.2BSD), then
accesses to that partition via the block device will use the blocksize
you set for the partition.
:dd if=/dev/wd3c of=/dev/null bs=64b
:
: tty da0 da1 wd0 wd2 wd3 cpu
: tin tout KB/t tps MB/s KB/t tps MB/s KB/t tps MB/s KB/t tps MB/s KB/t tps MB/s us ni sy in id
: 8 109 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 2.05 1290 2.59 2 0 18 1 79
: 4 109 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 2.00 1588 3.10 2 0 20 4 74
: 16 110 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 2.00 1602 3.13 2 0 25 3 70
For the raw device no reblocking is done, the block size you
give dd is what dd will attempt to hand to the raw device.
:dd if=/dev/rwd3c of=/dev/null bs=64b
: 0 108 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 32.00 312 9.74 2 0 5 1 92
: 148 109 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 32.00 311 9.71 3 0 4 0 93
: 371 108 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 32.00 312 9.74 5 0 11 1 83
:> If you apply the mmap() patches found on
:> http://www.backplane.com/FreeBSD4/ and do an mmap based test, you will
:> find that the overhead narrows considerably.
:
:Hmmm. I tried running with 2kB blocks, and the results weren't what I
:expected. I need to scratch my head some more. Here's what I got:
The patches only apply to mmap. dd does not use mmap to read the
input file.
:dd if=/dev/rwd3c of=/dev/null bs=4b count=10000
:
: 1 109 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 2.00 3891 7.60 2 0 28 4 67
: 0 109 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 2.00 3889 7.60 4 0 24 7 66
:
:dd if=/dev/wd3c of=/dev/null bs=4b count=10000
:
: 193 325 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 2.10 143 0.29 4 0 2 1 93
:2113 342 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 0.00 0 0.00 2.76 162 0.44 5 0 4 1 90
:
:> There is nothing sinister going on here, buffer copying eats cpu
:> plain and simple.
:
:The primary effect here isn't buffer copying.
:
:Greg
:--
:See complete headers for address, home page and phone numbers
:finger grog@lemis.com for PGP public key
:
-Matt
Matthew Dillon
<dillon@backplane.com>
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe cvs-all" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?199909200611.XAA77678>