On Tue, Dec 09, 2008 at 04:38:07PM -0600, James Bottomley wrote: > On Tue, 2008-12-09 at 15:21 -0700, Matthew Wilcox wrote: > > The performance penalty is certainly measurable. It's about 1 microsecond > > per request extra to go from userspace -> scsi -> libata -> driver > > than it is to go from userspace -> scsi -> driver. If you issue 400 > > commands per second (as you might do with a 15k RPM SCSI drive), that's > > 400 microseconds. If you issue 10,000 commands per second (as you might > > do with an SSD), that's 10ms of additional CPU time spent in the kernel > > per second (or 1%). > > Um, not quite. What you're talking about is increased latency. It's Tsk. I was quite clear I wasn't talking about latency or bandwidth. I was talking about the amount of CPU used to keep a device busy. > not cumulative because we use TCQ (well mostly). The question is really > how it impacts the benchmarks, which are mostly throughput based (and > really, our block layer trades latency for throughput anyway, so it's > not clear what the impact really is). If 1% of CPU is being used by the kernel, that's 1% of CPU not available for the user application (or alternatively an extra centisecond the CPU could be in a low-power state if you're not CPU-bound). > > (OK, I haven't measured the overhead of the *SCSI* layer, I've measured > > the overhead of the *libata* layer. I think the point here is that you > > can't measure the difference at a macro level unless you're sending a > > lot of commands.) > > Perhaps one of the things we should agree on is exactly how we want to > measure things like this. Making the layering thinner for less latency > is usually good ... unless there are other tradeoffs. I think not > forcing ata disks to go through SCSI will probably be tradeoff free, but > we need to make sure it is. That would certainly be a good idea. I don't think we have a consensus about what we should be measuring yet ;-) -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step." -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
