Date: Fri, 30 May 1997 12:42:20 +0200 From: Stefan Esser <se@FreeBSD.ORG> To: Poul-Henning Kamp <phk@dk.tfs.com> Cc: Bruce Evans <bde@zeta.org.au>, current@FreeBSD.ORG Subject: Re: disk cache challenged by small block sizes Message-ID: <19970530124220.36367@x14.mi.uni-koeln.de> In-Reply-To: <373.864918382@critter.dk.tfs.com>; from Poul-Henning Kamp on Thu, May 29, 1997 at 05:06:22PM %2B0200 References: <199705291228.WAA11504@godzilla.zeta.org.au> <373.864918382@critter.dk.tfs.com>
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On May 29, Poul-Henning Kamp <phk@dk.tfs.com> wrote: > > >ufs with a block size of 4K was about the same speed as ext2fs with a block > >size of 4K. ufs with a block size of 8K was significantly (25%) slower. > =========================================================== > > Uhm, isn't that rather obvious ? Ahemm ??? > The chances of being able to do one DMA into two (physically) consecutive > pages are very very slim as far as I can tell, so you will generally get > better performance when you do it in page size chunks ? Sure! > In other words, drivers should be optimized for two back-to-back 4K > transactions to different pages, not for 8k transactions to physically > contiguous pages. Well, sorry, hmmm, ... When did you write a bus-master SCSI driver, last time ? ;-) For a "n" page transfer, n+1 scatter/gather table entries are provided. This allows for the transfer to start at an arbitrary byte offset into the first page, and covers n-1 full pages, and possibly a final partial one ... Regards, STefan
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