Date: Sat, 8 Apr 1995 20:21:11 +1000 From: Bruce Evans <bde@zeta.org.au> To: freebsd-current@FreeBSD.org, taob@gate.sinica.edu.tw Subject: Re: Disk performance Message-ID: <199504081021.UAA24575@godzilla.zeta.org.au>
next in thread | raw e-mail | index | archive | help
> My feeling is that it should have been lower and not anywhere >close to 100% usage. Sending out 366 I/O requests to a SCSI device >and waiting for them to return did not seem to warrant a 50% busy >state with a 100-MHz processor on a 33-MHz bus. I gather this is >where IDE drives fare much worse? Actually only 366/8 i/o requests are sent to SCSI devices. Iozone does huge sequential i/o's on which clustering works perfectly, so file data is always read and written 64K at a time (not 8K for a file system with a block size of 8K). Normal file accesses aren't as sequential as for iozone, so clustering doesn't work so well. Normal file accesses are often 8 times as slow as for iozone for this and other reasons (seeking...) :-(. For IDE, drives, sending out 366/8 I/O requests is much faster, but "waiting" for them to return actually requires handling up to 366*16 interrupts (one for each sector) and copying 512 bytes or more per interrupt. Interrupt overhead is about 5usec/interrupt on a P90 and copying overhead is about 155usec/sector for the old IDE interface on all systems. Bruce
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?199504081021.UAA24575>