Date: Sat, 22 Apr 2000 01:51:54 +1000 From: Andrew Morton Subject: Re: IDE drives and unmaskirq [ I cut lkml - I've been posting too much today :) ] Martijn van Oosterhout wrote: > > Odd, both my hard disks are UDMA. You're saying unmaskirq has > no effect on UDMA systems? I have a patch which measures interrupt latencies. It tells you how long the kernel spends with interrupts disabled: where they were turned off, where they were turned on, min, max, avg number of microseconds. With UDMA66 drives: hdparm -u0 -d0 (interrupts masked, PIO) Count Min Max Avg ide-disk.c:451 -> ll_rw_blk.c:277 12 825.17 2350.14 1268.35 ide-disk.c:451 -> ide.c:1620 591 818.82 2172.18 1138.26 ide.c:1531 -> ide.c:1620 7407 12.13 2147.32 1519.29 ide.c:1531 -> ide.c:1296 721 9.51 1930.11 236.73 ll_rw_blk.c:271 -> ide.c:1296 2912 13.82 27.89 16.56 ... hdparm -t says 4.1 MB/sec (The first traversal seems a very odd place to reenable...) ------- hdparm -u0 -d1 (interrupts masked, DMA) ide.c:1531 -> ide.c:1620 5230 11.13 90.76 26.55 ide.c:1531 -> ide.c:1296 484 11.53 73.15 17.15 ll_rw_blk.c:271 -> ide.c:1296 5232 13.79 28.07 16.51 ide.c:1298 -> ll_rw_blk.c:277 5232 7.33 14.40 8.23 ... hdparm -t says 21 MB/sec ------ hdparm -u1 -d0 (interrupts unmasked, PIO) ide.c:522 -> ide.c:531 8631 .81 32.30 1.93 ll_rw_blk.c:271 -> ide.c:1296 3921 13.82 28.94 16.52 ide.c:497 -> ide.c:506 14388 1.07 24.79 6.07 ide.c:1298 -> ll_rw_blk.c:277 3921 7.33 14.76 8.15 ... hdparm -t says 4.2 MB/sec -------- hdparm -u1 -d1 (interrupts unmasked, DMA) ll_rw_blk.c:271 -> ide.c:1296 3950 13.78 26.74 16.43 ide.c:1713 -> ide.c:1296 1 23.61 23.61 23.61 ide.c:497 -> ide.c:506 14623 .99 23.22 5.06 ide.c:1298 -> ll_rw_blk.c:277 3950 7.35 15.72 8.11 ... hdparm -t says 22 MB/sec So you can see that even with DMA, 'hdparm -u1' cuts the max interrupt-disable time from 90 to 27 microseconds. I somehow doubt if that will affect interactive perceptions, but the two milliseconds for PIO/masked will affect audio, for example.