Date: Mon, 18 Nov 2002 13:05:31 +0100 From: Jens Axboe To: Steve Lord Cc: Linux Kernel Subject: Re: SCSI I/O performance problems when CONFIG_HIGHIO is off Message-ID: <20021118120531.GC839@suse.de> References: <1037392310.13531.419.camel@jen.americas.sgi.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1037392310.13531.419.camel@jen.americas.sgi.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 15 2002, Steve Lord wrote: > Jens, > > As you know, for the last week or so I have been battling some > performance issues in XFS and 2.4.20-rc1. Well, we finally found > the culprit back in 2.4.20-pre2. > > When the block highmem patch was included, it added highmem_io to the > scsi controller structure. This can only ever be set to one if > CONFIG_HIGHIO is set. Yet there are several spots in the scsi > code which test based on its value regardless. > > /* > * we really want to use sg even for a single segment request, > * however some people just cannot be bothered to write decent > * driver code so we can't risk to break somebody making the > * assumption that sg requests will always contain at least 2 > * segments. if the driver is 32-bit dma safe, then use sg for > * 1 entry anyways. if not, don't rely on the driver handling this > * case. > */ > if (count == 1 && !SCpnt->host->highmem_io) { > this_count = req->current_nr_sectors; > goto single_segment; > } Steve, Something isn't quite making sense. If we go over every single instance of checking ->highmem_io, they all look sane (ie checking on non-highmem setup must yield 0). So that part looks good. However, I think a typo snuck in there, in exactly the spot you pasted above. Could you try 2.4.20-rc2 with this patch applied? ===== drivers/scsi/scsi_merge.c 1.9 vs edited ===== --- 1.9/drivers/scsi/scsi_merge.c Mon Sep 16 09:25:10 2002 +++ edited/drivers/scsi/scsi_merge.c Mon Nov 18 13:04:41 2002 @@ -835,7 +835,7 @@ * case. */ if (count == 1 && !SCpnt->host->highmem_io) { - this_count = req->current_nr_sectors; + this_count = req->nr_sectors; goto single_segment; } -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/