I'd like to think that, too - probably true for the Atari TT SCSI case (can do scatter-gather, can do more than one command per LUN). Worse for the Falcon SCSI which is the only one I can test (no scatter-gather, one command per LUN, interrupt shared with IDE and IDE driver locked out while SCSI command handled). But that only affects balancing of I/O between IDE and SCSI drivers. Is that what you are worried about, Alan? Happy to test whether limiting max_sectors makes a difference in the DMA case. Cheers, Michael On Wed, Dec 23, 2015 at 2:47 AM, Finn Thain <fthain@xxxxxxxxxxxxxxxxxxx> wrote: > > On Tue, 22 Dec 2015, One Thousand Gnomes wrote: > >> On Tue, 22 Dec 2015 12:18:44 +1100 Finn Thain >> <fthain@xxxxxxxxxxxxxxxxxxx> wrote: >> >> > Because of the rudimentary design of the chip, it is necessary to poll >> > the SCSI bus signals during PIO and this tends to hog the CPU. The >> > driver will accept new commands while others execute, and this causes >> > a soft lockup because the workqueue item will not terminate until the >> > issue queue is emptied. >> > >> > When exercising dmx3191d using sequential IO from dd, the driver is >> > sent 512 KiB WRITE commands and 128 KiB READs. For a PIO transfer, the >> > rate is is only about 300 KiB/s, so these are long-running commands. >> > And although PDMA may run at several MiB/s, interrupts are disabled >> > for the duration of the transfer. >> > >> > Fix the unresponsiveness and soft lockup issues by calling >> > cond_resched() after each command is completed and by limiting >> > max_sectors for drivers that don't implement real DMA. >> >> Is there a reason for not doing some limiting in the DMA case too. A >> 512K write command even with DMA on a low end 68K box introduces a >> second of latency before another I/O can be scheduled ? > > The DMA case is the atari_scsi case. I'd like to think that atari_scsi > would have only the latency issues that might be expected from any SCSI-2 > host adapter driver. > > Unlike PDMA, interrupts are not disabled for these DMA transfers. Note > that this patch isn't really relevant to DMA, because the main loop > iterates only when done == 0, that is, !hostdata->dmalen. > > -- > >> >> Alan > -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
