On Fri, Apr 25, 2025 at 01:23:16PM +0800, Ming Lei wrote: > On Thu, Apr 24, 2025 at 12:07:32PM -0700, Caleb Sander Mateos wrote: > > On Thu, Apr 24, 2025 at 11:58 AM Ofer Oshri <ofer@xxxxxxxxxx> wrote: > > > > > > > > > > > > ________________________________ > > > From: Caleb Sander Mateos <csander@xxxxxxxxxxxxxxx> > > > Sent: Thursday, April 24, 2025 9:28 PM > > > To: Ofer Oshri <ofer@xxxxxxxxxx> > > > Cc: linux-block@xxxxxxxxxxxxxxx <linux-block@xxxxxxxxxxxxxxx>; ming.lei@xxxxxxxxxx <ming.lei@xxxxxxxxxx>; axboe@xxxxxxxxx <axboe@xxxxxxxxx>; Jared Holzman <jholzman@xxxxxxxxxx>; Yoav Cohen <yoav@xxxxxxxxxx>; Guy Eisenberg <geisenberg@xxxxxxxxxx>; Omri Levi <omril@xxxxxxxxxx> > > > Subject: Re: ublk: RFC fetch_req_multishot > > > > > > External email: Use caution opening links or attachments > > > > > > > > > On Thu, Apr 24, 2025 at 11:19 AM Ofer Oshri <ofer@xxxxxxxxxx> wrote: > > > > > > > > Hi, > > > > > > > > Our code uses a single io_uring per core, which is shared among all block devices - meaning each block device on a core uses the same io_uring. > > > > > > > > Let’s say the size of the io_uring is N. Each block device submits M UBLK_U_IO_FETCH_REQ requests. As a result, with the current implementation, we can only support up to P block devices, where P = N / M. This means that when we attempt to support block device P+1, it will fail due to io_uring exhaustion. > > > > > > What do you mean by "size of the io_uring", the submission queue size? > > > Why can't you submit all P * M UBLK_U_IO_FETCH_REQ operations in > > > batches of N? > > > > > > Best, > > > Caleb > > > > > > N is the size of the submission queue, and P is not fixed and unknown at the time of ring initialization.... > > > > I don't think it matters whether P (the number of ublk devices) is > > known ahead of time or changes dynamically. My point is that you can > > submit the UBLK_U_IO_FETCH_REQ operations in batches of N to avoid > > exceeding the io_uring SQ depth. (If there are other operations > > potentially interleaved with the UBLK_U_IO_FETCH_REQ ones, then just > > submit each time the io_uring SQ fills up.) Any values of P, M, and N > > should work. Perhaps I'm misunderstanding you, because I don't know > > what "io_uring exhaustion" refers to. > > > > Multishot ublk io_uring operations don't seem like a trivial feature > > to implement. Currently, incoming ublk requests are posted to the ublk > > server using io_uring's "task work" mechanism, which inserts the > > io_uring operation into an intrusive linked list. If you wanted a > > single ublk io_uring operation to post multiple completions, it would > > need to allocate some structure for each incoming request to insert > > into the task work list. There is also an assumption that the ublk > > io_uring operations correspond 1-1 with the blk-mq requests for the > > ublk device, which would be broken by multishot ublk io_uring > > operations. > > For delivering ublk io command to ublk server, I feel multishot can be > used in the following way: > > - use IORING_OP_READ_MULTISHOT to read from ublk char device, do it for > each queue, queue id may be passed via offset > > - block in ublk_ch_read_iter() if nothing comes from this queue of the > ublk block device > > - if any ublk block io comes, fill `ublksrv_io_desc` in mmapped area, and > push the 'tag' to the read ring buffer(provided buffer) > > - wakeup the read IO after one whole IO batch is done > > For commit ublk io command result to ublk driver, it can be similar with > delivering by writing 'tag' to ublk char device via IORING_OP_WRITE_FIXED or > IORING_OP_WRITE, still per queue via ring_buf approach, but need one mmapped > buffer for storing the io command result, 4 bytes should be enough for each io. > > With the above way: > > - use read/write to deliver io command & commit io command result, so > single read/write replaces one batch of uring_cmd > > - needn't uring command any more, big security_uring_cmd() cost can be avoided > > - memory footprint is reduced a lot, no extra uring_cmd for each IO > > - extra task work scheduling is avoided > > - Probably uring exiting handling can be simplified too. > > > Sounds like ublk 2.0 prototype, :-) I have been working towards this direction: https://github.com/ming1/linux/commits/ublk2-cmd-batch/ by adding three new batch commands, all are per-queue: `UBLK_U_IO_FETCH_IO_CMDS` - multishot with provided buffer - issued once, CQE is posted after new io/io batch is coming by filling io tag into the provided buffer - re-issue after the whole buffer is used up, so issue cost is reduced - multiple `UBLK_U_IO_FETCH_IO_CMDS` are allowed to be issued concurrently from different task contexts for supporting load balance - each `UBLK_U_IO_FETCH_IO_CMDS` can carry 'priority' info for supporting prioritized schedule, not done yet, should be easier to implement `UBLK_U_IO_COMMIT_IO_CMDS` - this command has a fixed buffer, in which io tag, io command result and other info(buf_index) for FETCH is provided, and multiple IOs or batch IO are covered `UBLK_U_IO_PREP_IO_CMDS`: batch version of `UBLK_IO_FETCH_REQ`, still has one fixed buffer for carrying io tag, info for fetch, similar with `UBLK_U_IO_COMMIT_IO_CMDS` In this way, lots of existing ublk constraint are relaxed: - any of the three command can be issued from any task context, there isn't per-io task or ubq_daemon limit any more. But AUTO_BUF_REG is one exception, which requires FETCH and COMMIT command are in same io_ring_ctx. - easier to support load balance, any IO commands fetched by the command of `UBLK_U_IO_FETCH_IO_CMDS` can be handled in the task for issuing UBLK_U_IO_FETCH_IO_CMDS - both FETCH and COMMIT are handled in batch way, communication cost is reduced. One drawback is that cost is added in client IO issue side(ublk_queue_rq() and ublk_queue_rqs()), goodness is that communication cost is reduced in ublk server side. Simple test running on one server shows that performance is good - kublk(`--batch --auto_zc -q 2` vs. `--auto_zc -q 2`): ~10% IOPS improvement The feature is still in very early stage, and any comments are welcome! Thanks, Ming
