On Mon, May 09, 2022 at 12:05:54PM +0800, Xiaoguang Wang wrote: > hi, > > > On Sat, May 07, 2022 at 12:20:17PM +0800, ZiyangZhang wrote: > >> On 2022/5/3 16:02, Ming Lei wrote: > >>> Hello Gabriel, > >>> > >>> CC linux-block and hope you don't mind, :-) > >>> > >>> On Mon, May 02, 2022 at 01:41:13PM -0400, Gabriel Krisman Bertazi wrote: > >>>> Hi Ming, > >>>> > >>>> First of all, I hope I didn't put you on the spot too much during the > >>>> discussion. My original proposal was to propose my design, but your > >>>> implementation quite solved the questions I had. :) > >>> I think that is open source, then we can put efforts together to make things > >>> better. > >>> > >>>> I'd like to follow up to restart the communication and see > >>>> where I can help more with UBD. As I said during the talk, I've > >>>> done some fio runs and I was able to saturate NBD much faster than UBD: > >>>> > >>>> https://people.collabora.com/~krisman/mingl-ubd/bw.png > >>> Yeah, that is true since NBD has extra socket communication cost which > >>> can't be efficient as io_uring. > >>> > >>>> I've also wrote some fixes to the initialization path, which I > >>>> planned to send to you as soon as you published your code, but I think > >>>> you might want to take a look already and see if you want to just squash > >>>> it into your code base. > >>>> > >>>> I pushed those fixes here: > >>>> > >>>> https://gitlab.collabora.com/krisman/linux/-/tree/mingl-ubd > >>> I have added the 1st fix and 3rd patch into my tree: > >>> > >>> https://github.com/ming1/linux/commits/v5.17-ubd-dev > >>> > >>> The added check in 2nd patch is done lockless, which may not be reliable > >>> enough, so I didn't add it. Also adding device is in slow path, and no > >>> necessary to improve in that code path. > >>> > >>> I also cleaned up ubd driver a bit: debug code cleanup, remove zero copy > >>> code, remove command of UBD_IO_GET_DATA and always make ubd driver > >>> builtin. > >>> > >>> ubdsrv part has been cleaned up too: > >>> > >>> https://github.com/ming1/ubdsrv > >>> > >>>> I'm looking into adding support for multiple driver queues next, and > >>>> should be able to share some patches on that shortly. > >>> OK, please post them on linux-block so that more eyes can look at the > >>> code, meantime the ubdsrv side needs to handle MQ too. > >>> > >>> Sooner or later, the single ubdsrv task may be saturated by copying data and > >>> io_uring command communication only, which can be shown by running io on > >>> ubd-null target. In my lattop, the ubdsrv cpu utilization is close to > >>> 90% when IOPS is > 500K. So MQ may help some fast backing cases. > >>> > >>> > >>> Thanks, > >>> Ming > >> Hi Ming, > >> > >> Now I am learning your userspace block driver(UBD) [1][2] and we plan to > >> replace TCMU by UBD as a new choice for implementing userspace bdev for > >> its high performance and simplicity. > >> > >> First, we have conducted some tests by fio and perf to evaluate UBD. > >> > >> 1) UBD achieves higher throughput than TCMU. We think TCMU suffers from > >> the complicated SCSI layer and does not support multiqueue. However > >> UBD is simply using io_uring passthrough and may support multiqueue in > >> the future.(Note that even with a single queue now , UBD outperforms TCMU) > > MQ isn't hard to support, and it is basically workable now: > > > > https://github.com/ming1/ubdsrv/commits/devel > > https://github.com/ming1/linux/commits/my_for-5.18-ubd-devel > > > > Just the affinity of pthread for each queue isn't setup yet. > > > >> 2) Some functions in UBD result in high CPU utilization and we guess > >> they also lower throughput. For example, ubdsrv_submit_fetch_commands() > >> frequently iterates on the array of UBD IOs and wastes CPU when no IO is > >> ready to be submitted. Besides, ubd_copy_pages() asks CPU to copy data > >> between bio vectors and UBD internal buffers while handling write and > >> read requests and it could be eliminated by supporting zero-copy. > > copy itself doesn't take much cpu, see the following trace: > > > > - 34.36% 3.73% ubd [kernel.kallsyms] [k] ubd_copy_pages.isra.0 ▒ > > - 30.63% ubd_copy_pages.isra.0 ▒ > > - 23.86% internal_get_user_pages_fast ▒ > > + 21.14% get_user_pages_unlocked ▒ > > + 2.62% lockless_pages_from_mm ▒ > > 6.42% ubd_release_pages.constprop.0 > > > > And we may provide option to allow to pin pages in the disk lifetime for avoiding > > the cost in _get_user_pages_fast(). > > > > zero-copy has to touch page table, and its cost may be expensive too. > > The big problem is that MM doesn't provide mechanism to support generic > > remapping kernel pages to userspace. > > > >> Second, I'd like to share some ideas on UBD. I'm not sure if they are > >> reasonable so please figure out my mistakes. > >> > >> 1) UBD issues one sqe to commit last completed request and fetch a new > >> one. Then, blk-mq's queue_rq() issues a new UBD IO request and completes > >> one cqe for the fetch command. We have evaluated that io_submit_sqes() > >> costs some CPU and steps of building a new sqe may lower throughput. > >> Here I'd like to give a new solution: never submit sqe but trump up a > >> cqe(with information of new UBD IO request) when calling queue_rq(). I > >> am inspired by one io_uring flag: IORING_POLL_ADD_MULTI, with which a > >> user issues only one sqe for polling an fd and repeatedly gets multiple > >> cqes when new events occur. Dose this solution break the architecture of > >> UBD? > > But each cqe has to be associated with one sqe, if I understand > > correctly. > Yeah, for current io_uring implementation, it is. But if io_uring offers below > helper: > void io_gen_cqe_direct(struct file *file, u64 user_data, s32 res, u32 cflags) > { > struct io_ring_ctx *ctx; > ctx = file->private_data; > > spin_lock(&ctx->completion_lock); > __io_fill_cqe(ctx, user_data, res, cflags); > io_commit_cqring(ctx); > spin_unlock(&ctx->completion_lock); > io_cqring_ev_posted(ctx); > } > > Then in ubd driver: > 1) device setup stage > We attach io_uring files and user_data to every ubd hard queue. > > 2) when blk-mq->queue_rq() is called. > io_gen_cqe_direct() will be called in ubd's queue_rq, and we put ubd io request's > qid and tag info into cqe's res field, then we don't need to issue sqe to fetch io cmds. The above way is actually anti io_uring design, and I don't think it may improve much since submitting UBD_IO_COMMIT_AND_FETCH_REQ is pretty lightweight. Also without re-submitting UBD_IO_COMMIT_AND_FETCH_REQ command, how can you commit io handling result from ubd server and ask ubd driver to complete io request? Thanks, Ming
