On Thu, 2005-12-08 at 21:46 +0300, Vladislav Bolkhovitin wrote: > - Scst has some performance advantages over stgt, at least, on > hardware targets, because it allows internal handling in SIRQ context as > well as doesn't need user space program, so it eliminates additional > context switches (at least 3 per command for WRITEs and 2 per command > for reads plus switches to user space daemon, probably, 2 per command). > 5 context switches per command looks too much for me, especially > considering how little work is done on each context. It means ~15000 > CS/sec on regular 200Mb/sec link with 64K block size. Additionally, > kernel only commands execution allows direct access to page cache, which > is GREAT performance improvement, because in architecture with user > space daemon the data should be copied several times between kernel and > user space. Or, do I miimpss anything? I do have to say that I consider operation in interrupt context (or even kernel context) to be a disadvantage. Compared with the response times that most arrays have to SCSI commands, the kernel context switch time isn't that significant. Additionally, it's perfectly possible for all of this to be done zero copy on the data. A user space target mmaps the data on its storage device and then does a SG_IO type scatter gather user virtual region pass to the underlying target infrastructure. We already have this demonstrated in the SG_IO path, someone just needs to come up with the correct implementation for a target path. The great advantage of doing SCSI state machines in user space is that you can prototype anything you want, and user space has much better state machine implementation (and debugging) tools available than the kernel does. James - : 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
