On Sun, Feb 20 2005, Pete Zaitcev wrote: > Hi, Jens: > > I think this question belongs to your domain, but please let me know > if I'm mistaken, so I can pursue this elsewhere. > > I encountered a strange performance anomaly. I do the following: > > <----- Plug USB key > [EMAIL PROTECTED] ~]# time dd if=/dev/uba of=/dev/null bs=10k count=10240 > 10240+0 records in > 10240+0 records out > > real 0m22.731s > user 0m0.004s > sys 0m0.345s > [EMAIL PROTECTED] ~]# > > <----- Remove and replug the USB key > [EMAIL PROTECTED] ~]# time dd if=/dev/uba1 of=/dev/null bs=10k count=10240 > 10240+0 records in > 10240+0 records out > > real 1m42.622s > user 0m0.005s > sys 0m1.518s > [EMAIL PROTECTED] ~]# > > So, reading from a partition of the same device is 5 times slower than > reading from the device itself. The question is, why? > > To the best of my knowledge, this does not occur with SCSI (usb-storage > and sd or sr). This hints strongly that the ub is not doing something > right, but what that can be? > > The ub takes the request processing machinery from Carmel exactly. I am > wondering if Carmel (sx8) exhibits any similar performance anomalies > (cc-ing to Jeff)
I can't explain why the replugging slows it down, maybe you were lucky to get contigious pages in the first case? As far as I can see, ub effectively disables merging by setting max hw/phys segment limit of 1. -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
