> Il giorno 18 dic 2018, alle ore 13:45, Paolo Valente <paolo.valente@xxxxxxxxxx> ha scritto: > > Hi Jens, > sorry for the following silly question, but maybe you can solve very > quickly a doubt for which I'd spend much more time investigating. > > While doing some tests with scsi_debug, I've just seen that (at least) > with direct I/O, the maximum number of pending I/O requests (at least > in the I/O schedulers) is equal, unexpectedly, to the queue depth of > the drive and not to > /sys/block/<dev>/queue/nr_requests > > For example, after: > > sudo modprobe scsi_debug max_queue=4 > > and with fio executed as follows: > > job: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=libaio, iodepth=20 > > I get this periodic trace, where four insertions are followed by four > completions, and so on, till the end of the I/O. This trace is taken > with none, but the result is the same with bfq. > > fio-20275 [001] d... 7560.655213: 8,48 I R 281088 + 8 [fio] > fio-20275 [001] d... 7560.655288: 8,48 I R 281096 + 8 [fio] > fio-20275 [001] d... 7560.655311: 8,48 I R 281104 + 8 [fio] > fio-20275 [001] d... 7560.655331: 8,48 I R 281112 + 8 [fio] > <idle>-0 [001] d.h. 7560.749868: 8,48 C R 281088 + 8 [0] > <idle>-0 [001] dNh. 7560.749912: 8,48 C R 281096 + 8 [0] > <idle>-0 [001] dNh. 7560.749928: 8,48 C R 281104 + 8 [0] > <idle>-0 [001] dNh. 7560.749934: 8,48 C R 281112 + 8 [0] > fio-20275 [001] d... 7560.750023: 8,48 I R 281120 + 8 [fio] > fio-20275 [001] d... 7560.750196: 8,48 I R 281128 + 8 [fio] > fio-20275 [001] d... 7560.750229: 8,48 I R 281136 + 8 [fio] > fio-20275 [001] d... 7560.750250: 8,48 I R 281144 + 8 [fio] > <idle>-0 [001] d.h. 7560.842510: 8,48 C R 281120 + 8 [0] > <idle>-0 [001] dNh. 7560.842551: 8,48 C R 281128 + 8 [0] > <idle>-0 [001] dNh. 7560.842556: 8,48 C R 281136 + 8 [0] > <idle>-0 [001] dNh. 7560.842562: 8,48 C R 281144 + 8 [0] > > Shouldn't the total number of pending requests reach > /sys/block/<dev>/queue/nr_requests ? > > The latter is of course equal to 8. > I forgot: I did this with a 4.18. Maybe this is something that has been changed/fixed? Thanks, Paolo > Thanks, > Paolo >
