difference between "lat" and "clat"
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Hi Jens, hi everyone, Well mails from my mail client seem to appear here. I have some questions regarding fio latencies. There are three kind of latencies described in README, from which fio 2.0.7 only displays clat and lat: slat = submission latency clat = completion latency lat = I/O completion latency Now I try to understand the README but I am not sure whether I really got the difference. I understand: slat: The time to submit the I/O. This is the time after which a syscall returns to userspace. On sync I/O this will include the completion of the request as sync I/O will wait for the request to land in pagecache. On asynchronous I/O this will be really fast. clat: This is the time to complete handling the I/O request. Then it is at least in the pagecache on buffered I/O. lat: The time it takes till the request has been processed. The HOWTO says: "This is the time from when IO leaves fio and when it gets completed." So how is that completion different from the other completion in clat? In fact values seem to be roughly the same: Reads: clat (usec): min=773 , max=104267 , avg=6333.91, stdev=4327.57 lat (usec): min=773 , max=104267 , avg=6334.10, stdev=4327.57 Writes: clat (usec): min=773 , max=104267 , avg=6333.91, stdev=4327.57 lat (usec): min=773 , max=104267 , avg=6334.10, stdev=4327.57 For: root@frisbie:~# cat randreadwrite.job [global] size=2g runtime=60 direct=1 [zufälliglesen] rw=randread [zufälligschreiben] rw=randwrite But even on buffered I/O this seems to be similar (same job without direct=1): Reads: clat (usec): min=189 , max=954818 , avg=5423.72, stdev=21529.85 lat (usec): min=190 , max=954818 , avg=5423.90, stdev=21529.85 Writes: clat (usec): min=3 , max=2391.5K, avg=180.47, stdev=8936.82 lat (usec): min=4 , max=2391.5K, avg=180.59, stdev=8936.82 So whats the difference here? Is lat = slat + clat? Why did you drop slat from output? I tried to enable it. But I get: root@frisbie:~# fio randreadwrite-buffered.job fio: failed parsing disable_slat=false fio: job global dropped root@frisbie:~# Is this another deprecated manpage option? And then I try to understand the clat new output: clat percentiles (usec): | 1.00th=[ 4], 5.00th=[ 5], 10.00th=[ 5], 20.00th=[ 5], | 30.00th=[ 5], 40.00th=[ 5], 50.00th=[ 5], 60.00th=[ 6], | 70.00th=[ 6], 80.00th=[ 6], 90.00th=[ 6], 95.00th=[ 7], | 99.00th=[ 12], 99.50th=, 99.90th=, 99.95th=, | 99.99th= What does the th mean? After reading documentation on "percentile_list" read this as follows: - 1% of the requests have taken 4 usec - 5% of the requests have taken 5 usec (does that include the 1% number? Seems so) - 10% of the requests have taken 5 usec (some issue with accuracy here? 5.something usec?) […] - 70% of the requests have taken 6 usec - 99,99% of the requests have taken less than 58112 usec Up to know I have looked at the most was "lat" - seems that seemed to be the most realistic value to me. Is there any point in giving much attention to clat from an admin point of view? I am inclined to omit it from output via "disable_clat" unless I see a case where I need this. Thanks, -- Martin Steigerwald - teamix GmbH - http://www.teamix.de gpg: 19E3 8D42 896F D004 08AC A0CA 1E10 C593 0399 AE90 -- To unsubscribe from this list: send the line "unsubscribe fio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html