Date: 25 Oct 1999 12:04:37 +0000 From: Randell Jesup <rjesup@wgate.com> To: "Michael Sinz" <Michael.Sinz@sinz.org> Cc: "Geoff Buckingham" <geoffb@chuggalug.clues.com>, "Gerard Roudier" <groudier@club-internet.fr>, "scsi@FreeBSD.ORG" <scsi@FreeBSD.ORG> Subject: Re: FreeBSD 3.2 / Slow SCSI Dell PowerEdge 4300 Message-ID: <ybu3duzodd6.fsf@jesup.eng.tvol.net.jesup.eng.tvol.net> In-Reply-To: "Michael Sinz"'s message of "Mon, 25 Oct 1999 08:40:43 -0400" References: <199910251239.IAA28752@vixen.sinz.org>
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"Michael Sinz" <Michael.Sinz@sinz.org> writes:
>>> For a general purpose server, one must assume that the special case of
>>> the I/O working out to be single threaded will not happen. Multiple things
>>> will be going on and the working set will be larger than the cache size.
>>> A bit of overhead added to the "simple" cases will make the general
>>> operation better. Benchmarks, however, may well show this as slower
>>> since some extra overhead had to be added. Benchmarks would need to
>>> become much more complex in order to show the real benefit or lack of
>>> benefit for any one technique.
>>>
>>Which brings us back to the question as to wether or not disabling TAGs for
>>WDE * is the correct thing to do? IMHO it is not.
>
>For the general case I would say that disabling TAGS is not worth the
>few specific case performance improvements (most of which are benchmark-only)
I must agree for a server that Tags are good in most cases. Even
for a single-user machine, I'm not certain I'd take a significant hit to
multitasking (multiple-accessor) disk performance to get a 5% improvement
to pure sequential IO. Now, there may be issues with the disksort
algorithm that is loosing the benefits of Tags for multi-threaded IO as
well; remember that most disksort algorithms were originally designed with
cacheless (and single-threaded) drives in mind. Have there been any good
ACM/etc papers on disksort and multi-threaded IO systems in the last few
years?
--
Randell Jesup, Worldgate Communications, ex-Scala, ex-Amiga OS team ('88-94)
rjesup@wgate.com
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