On Wed, 2013-09-18 at 01:41 -0500, Alireza Haghdoost wrote: > Hi > > I am working on a high throughput and low latency application which > does not tolerate block layer overhead to send IO request directly to > fiber channel lower layer SCSI driver. I used to work with libaio but > currently I am looking for a way to by pass the block layer and send > SCSI commands from the application layer directly to the SCSI driver > using /dev/sgX device and ioctl() system call. > > I have noticed that sending IO request through sg device even with > nonblocking and direct IO flags is quite slow and does not fill up > lower layer SCSI driver TCQ queue. i.e IO depth or > /sys/block/sdX/in_flight is always ZERO. Therefore the application > throughput is even lower that sending IO request through block layer > with libaio and io_submit() system call. In both cases I used only one > IO context (or fd) and single threaded. > > I have noticed that some well known benchmarking tools like fio does > not support IO depth for sg devices as well. Therefore, I was > wondering if it is feasible to bypass block layer and achieve higher > throughput and lower latency (for sending IO request only). > > > Any comment on my issue is highly appreciated. > > FYI, you've got things backward as to where the real overhead is being introduced. The block layer / aio overhead is minimal compared to the overhead introduced by the existing scsi_request_fn() logic, and extreme locking contention between request_queue->queue_lock and scsi_host->host_lock that are accessed/released multiple times per struct scsi_cmnd dispatch. This locking contention and other memory allocations currently limit per struct scsi_device performance with small block random IOPs to ~250K vs. ~1M with raw block drivers providing their own make_request() function. FYI, there is an early alpha scsi-mq prototype that bypasses the scsi_request_fn() junk all together, that is able to reach small block IOPs + latency that is comparable to raw block drivers. Only a handful of LLDs have been converted to run with full scsi-mq pre-allocation thus far, and the code is considered early, early alpha. It's the only real option for SCSI to get anywhere near raw block driver performance + latency, but is still quite a ways off mainline. --nab -- 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
