Hi Tejun, On Tue, Dec 9, 2008 at 6:47 PM, Tejun Heo <htejun@xxxxxxxxx> wrote: ... >> That's the whole point of SSDs (lots of small, random IO). > > But on many workloads, filesystems manage to colocate what belongs > together and with little help from read ahead and block layer we > manage to dish out decently sized requests. True. And plenty of applications use a database which can't co-locate the data. Read ahead for random IO just wastes BW and CPU cycles. > It will be great to serve > 4k requests as fast as we can but whether that should be (or rather > how much) the focal point of optimization is a slightly different > problem. "How much the focal point" is a fair question. If someone can produce a super efficient SATA or SAS storage controller, I'd think it would matter more. ... >> Willy presented how he measured SCSI stack at LSF2008. ISTR he was >> advised to use oprofile in his test application so there is probably >> an updated version of these slides: >> http://iou.parisc-linux.org/lsf2008/IO-latency-Kristen-Carlson-Accardi.pdf > > Ah... okay, with ram low level driver. Right. that's alot faster than any SSD. But it's a convenient way to get consistent, precise numbers for workloads that can be scaled down to fit into RAM. ... >> Maybe you are counting instructions and not cycles? Every cache miss >> is 200-300 cycles (say 100ns). When running multiple threads, we will >> miss on nearly every spinlock acquisition and probably on several data >> accesses. 1 microsecond isn't alot when counting this way. > > Yeah, ata uses its own locking and the qc allocation does atomic > bitops for each bit for no good reason which can hurt for very hi-ops > with NCQ tags filled up. If serving 4k requests as fast as possible > is the goal, I'm not really sure the current SCSI or ATA commands are > the best suited ones. Both SCSI and ATA are focused on rotating media > with seek latency I think existing File Systems and block IO schedulers (except NOOP) are tuned for rotating media and access patterns that benefit this media the most. > and thus have SG on the host bus side in mode cases > but never on the device side. SG == scatter-gather? I'm not sure why that is specific to rotating media. Or is this referring to "SCSI-generic" pass through? In any case, only traversing one fewer layers (SCSI or libata) in block code path would help serve 4k requests more efficiently. > If getting the maximum random scattered > access throughput is a must, the best way would be adding a SG r/w > commands to ATA and adapt our storage stack accordingly. I don't think everyone wants to throw out the entire stack. But adding a passthrough for ATA and connecting that to FUSE might be a performant alternative. thanks, grant > Thanks. > > -- > tejun > -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
