Re: difference between "lat" and "clat"

[Martin Steigerwald <ms@teamix.de>]

Am Mittwoch, 9. Mai 2012 schrieb Martin Steigerwald:
> 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?
[…]
> 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?

Could be (from another job with iodepth 64)

write:
slat (usec): min=5 , max=753280 , avg=190.49, stdev=8787.41
clat (usec): min=677 , max=2178.2K, avg=317831.81, stdev=282569.45
lat (usec): min=715 , max=2178.2K, avg=318023.20, stdev=282631.85

read:
slat (usec): min=5 , max=139 , avg=22.73, stdev= 5.52
clat (msec): min=8 , max=1756 , avg=203.10, stdev=174.58
lat (msec): min=8 , max=1756 , avg=203.13, stdev=174.58

Hmmm, on read averages don't add up.

> Why did you drop slat from output?

Forget this one. It obviously is there.

Ciao,
-- 
Martin Steigerwald - teamix GmbH - http://www.teamix.de
gpg: 19E3 8D42 896F D004 08AC A0CA 1E10 C593 0399 AE90