For operations that matter in this context (read, write), there can be multiple outstanding tags. A while back rsc implemented fcp, partly to prove this point.
On Wed, Oct 10, 2018 at 2:54 PM Steven Stallion <sstall...@gmail.com> wrote: > As the guy who wrote the majority of the code that pushed those 1M 4K > random IOPS erik mentioned, this thread annoys the shit out of me. You > don't get an award for writing a driver. In fact, it's probably better > not to be known at all considering the bloody murder one has to commit > to marry hardware and software together. > > Let's be frank, the I/O handling in the kernel is anachronistic. To > hit those rates, I had to add support for asynchronous and vectored > I/O not to mention a sizable bit of work by a co-worker to properly > handle NUMA on our appliances to hit those speeds. As I recall, we had > to rewrite the scheduler and re-implement locking, which even Charles > Forsyth had a hand in. Had we the time and resources to implement > something like zero-copy we'd have done it in a heartbeat. > > In the end, it doesn't matter how "fast" a storage driver is in Plan 9 > - as soon as you put a 9P-based filesystem on it, it's going to be > limited to a single outstanding operation. This is the tyranny of 9P. > We (Coraid) got around this by avoiding filesystems altogether. > > Go solve that problem first. > On Wed, Oct 10, 2018 at 12:36 PM <cinap_len...@felloff.net> wrote: > > > > > But the reason I want this is to reduce latency to the first > > > access, especially for very large files. With read() I have > > > to wait until the read completes. With mmap() processing can > > > start much earlier and can be interleaved with background > > > data fetch or prefetch. With read() a lot more resources > > > are tied down. If I need random access and don't need to > > > read all of the data, the application has to do pread(), > > > pwrite() a lot thus complicating it. With mmap() I can just > > > map in the whole file and excess reading (beyond what the > > > app needs) will not be a large fraction. > > > > you think doing single 4K page sized reads in the pagefault > > handler is better than doing precise >4K reads from your > > application? possibly in a background thread so you can > > overlap processing with data fetching? > > > > the advantage of mmap is not prefetch. its about not to do > > any I/O when data is already in the *SHARED* buffer cache! > > which plan9 does not have (except the mntcache, but that is > > optional and only works for the disk fileservers that maintain > > ther file qid ver info consistently). its *IS* really a linux > > thing where all block device i/o goes thru the buffer cache. > > > > -- > > cinap > > > >
