On Apr 12, 2011, at 11:32 AM, Badari Pulavarty wrote:
> Hi,
>
> We recently ran into serious performance issue with NFS client.
> It turned out that its due to lack of readv/write support for
> NFS (O_DIRECT) client.
>
> Here is our use-case:
>
> In our cloud environment, our storage is over NFS. Files
> on NFS are passed as a blockdevices to the guest (using
> O_DIRECT). When guest is doing IO on these block devices,
> they will end up as O_DIRECT writes to NFS (on KVM host).
>
> QEMU (on the host) gets a vector from virtio-ring and
> submits them. Old versions of QEMU, linearized the vector
> it got from KVM (copied them into a buffer) and submits
> the buffer. So, NFS client always received a single buffer.
>
> Later versions of QEMU, eliminated this copy and submits
> a vector directly using preadv/pwritev().
>
> NFS client loops through the vector and submits each
> vector as separate request for each IO < wsize. In our
> case (negotiated wsize=1MB), for 256K IO - we get 64
> vectors, each 4K. So, we end up submitting 64 4K FILE_SYNC IOs.
> Server end up doing each 4K synchronously. This causes
> serious performance degrade. We are trying to see if the
> performance improves if we convert IOs to ASYNC - but
> our initial results doesn't look good.
>
> readv/writev support NFS client for all possible cases is
> hard. Instead, if all vectors are page-aligned and
> iosizes page-multiple - it fits the current code easily.
> Luckily, QEMU use-case fits these requirements.
>
> Here is the patch to add this support. Comments ?
Restricting buffer alignment requirements would be an onerous API change, IMO.
If the NFS write path is smart enough not to set FILE_SYNC when there are multiple segments to write, then the problem should be mostly fixed. I think Jeff Layton already has a patch that does this.
>
> Thanks,
> Badari
>
> Special readv/writev support for NFS O_DIRECT. Currently NFS
> client O_DIRECT read/write iterates over individual elements
> in the vector and submits the IO and waits for them to finish.
> If individual IOsize is < r/wsize, each IO will be FILE_SYNC.
> Server has to finish individual smaller IOs synchronous. This
> causes serious performance degrade.
>
> This patch is a special case to submit larger IOs from the client
> only if all the IO buffers are page-aligned and each individual
> IOs are page-multiple + total IO size is < r/wsize.
>
> This is the common case for QEMU usage.
>
> Signed-off-by: Badari Pulavarty <pbadari@xxxxxxxxxx>
> ---
> fs/nfs/direct.c | 259 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 259 insertions(+)
>
> Index: linux-2.6.38.2/fs/nfs/direct.c
> ===================================================================
> --- linux-2.6.38.2.orig/fs/nfs/direct.c 2011-04-12 10:56:29.374266292 -0400
> +++ linux-2.6.38.2/fs/nfs/direct.c 2011-04-12 11:01:21.883266310 -0400
> @@ -271,6 +271,52 @@ static const struct rpc_call_ops nfs_rea
> .rpc_release = nfs_direct_read_release,
> };
>
> +static int nfs_direct_check_single_io(struct nfs_direct_req *dreq,
> + const struct iovec *iov,
> + unsigned long nr_segs, int rw)
> +{
> + unsigned long seg;
> + int pages = 0;
> + struct nfs_open_context *ctx = dreq->ctx;
> + struct inode *inode = ctx->path.dentry->d_inode;
> + size_t size;
> +
> +
> + /* If its a single vector - use old code */
> + if (nr_segs == 1)
> + return pages;
> + /*
> + * Check to see if all IO buffers are page-aligned and
> + * individual IO sizes are page-multiple.
> + * If so, we can submit them in a single IO.
> + * Otherwise, use old code
> + */
> + for (seg = 0; seg < nr_segs; seg++) {
> + unsigned long user_addr = (unsigned long)iov[seg].iov_base;
> +
> + if ((user_addr & (PAGE_SIZE - 1)) ||
> + (iov[seg].iov_len & (PAGE_SIZE - 1))) {
> + pages = 0;
> + break;
> + }
> + pages += (iov[seg].iov_len >> PAGE_SHIFT);
> + }
> +
> + if (rw)
> + size = NFS_SERVER(inode)->wsize;
> + else
> + size = NFS_SERVER(inode)->rsize;
> +
> + /*
> + * If IOsize > wsize/rsize, we need to split IOs into r/wsize anyway
> + * - fall back to old code.
> + */
> + if (pages > (size >> PAGE_SHIFT))
> + pages = 0;
> +
> + return pages;
> +}
> +
> /*
> * For each rsize'd chunk of the user's buffer, dispatch an NFS READ
> * operation. If nfs_readdata_alloc() or get_user_pages() fails,
> @@ -385,6 +431,101 @@ static ssize_t nfs_direct_read_schedule_
> return result < 0 ? (ssize_t) result : -EFAULT;
> }
>
> +/*
> + * Special Case: Efficient readv() support - only if all the IO buffers
> + * in the vector are page-aligned and IO sizes are page-multiple
> + * + total IO size is < rsize. We can map all of the vectors together
> + * and submit them in a single IO instead of operating on single entry
> + * in the vector.
> + */
> +static ssize_t nfs_direct_read_schedule_single_io(struct nfs_direct_req *dreq,
> + const struct iovec *iov,
> + unsigned long nr_segs,
> + int pages,
> + loff_t pos)
> +{
> + struct nfs_open_context *ctx = dreq->ctx;
> + struct inode *inode = ctx->path.dentry->d_inode;
> + unsigned long user_addr = (unsigned long)iov->iov_base;
> + size_t bytes = pages << PAGE_SHIFT;
> + struct rpc_task *task;
> + struct rpc_message msg = {
> + .rpc_cred = ctx->cred,
> + };
> + struct rpc_task_setup task_setup_data = {
> + .rpc_client = NFS_CLIENT(inode),
> + .rpc_message = &msg,
> + .callback_ops = &nfs_read_direct_ops,
> + .workqueue = nfsiod_workqueue,
> + .flags = RPC_TASK_ASYNC,
> + };
> + unsigned int pgbase;
> + int result;
> + int seg;
> + int mapped = 0;
> + int nr_pages;
> + ssize_t started = 0;
> + struct nfs_read_data *data;
> +
> + result = -ENOMEM;
> + data = nfs_readdata_alloc(pages);
> + if (unlikely(!data))
> + return result;
> +
> + pgbase = user_addr & ~PAGE_MASK;
> +
> + for (seg = 0; seg < nr_segs; seg++) {
> + user_addr = (unsigned long)iov[seg].iov_base;
> + nr_pages = iov[seg].iov_len >> PAGE_SHIFT;
> +
> + down_read(¤t->mm->mmap_sem);
> + result = get_user_pages(current, current->mm, user_addr,
> + nr_pages, 1, 0, &data->pagevec[mapped],
> + NULL);
> + up_read(¤t->mm->mmap_sem);
> + if (result < 0) {
> + /* unmap what is done so far */
> + nfs_direct_release_pages(data->pagevec, mapped);
> + nfs_readdata_free(data);
> + return result;
> + }
> + mapped += result;
> + }
> +
> + get_dreq(dreq);
> +
> + data->req = (struct nfs_page *) dreq;
> + data->inode = inode;
> + data->cred = msg.rpc_cred;
> + data->args.fh = NFS_FH(inode);
> + data->args.context = ctx;
> + data->args.offset = pos;
> + data->args.pgbase = pgbase;
> + data->args.pages = data->pagevec;
> + data->args.count = bytes;
> + data->res.fattr = &data->fattr;
> + data->res.eof = 0;
> + data->res.count = bytes;
> + nfs_fattr_init(&data->fattr);
> +
> + task_setup_data.task = &data->task;
> + task_setup_data.callback_data = data;
> + msg.rpc_argp = &data->args;
> + msg.rpc_resp = &data->res;
> + NFS_PROTO(inode)->read_setup(data, &msg);
> +
> + task = rpc_run_task(&task_setup_data);
> + if (IS_ERR(task))
> + goto out;
> + rpc_put_task(task);
> +
> + started += bytes;
> +out:
> + if (started)
> + return started;
> + return result < 0 ? (ssize_t) result : -EFAULT;
> +}
> +
> static ssize_t nfs_direct_read_schedule_iovec(struct nfs_direct_req *dreq,
> const struct iovec *iov,
> unsigned long nr_segs,
> @@ -396,6 +537,16 @@ static ssize_t nfs_direct_read_schedule_
>
> get_dreq(dreq);
>
> + result = nfs_direct_check_single_io(dreq, iov, nr_segs, 0);
> + if (result) {
> + result = nfs_direct_read_schedule_single_io(dreq, iov, nr_segs,
> + result, pos);
> + if (result < 0)
> + goto out;
> + requested_bytes += result;
> + pos += result;
> + goto out;
> + }
> for (seg = 0; seg < nr_segs; seg++) {
> const struct iovec *vec = &iov[seg];
> result = nfs_direct_read_schedule_segment(dreq, vec, pos);
> @@ -416,6 +567,7 @@ static ssize_t nfs_direct_read_schedule_
> return result < 0 ? result : -EIO;
> }
>
> +out:
> if (put_dreq(dreq))
> nfs_direct_complete(dreq);
> return 0;
> @@ -821,6 +973,102 @@ static ssize_t nfs_direct_write_schedule
> return result < 0 ? (ssize_t) result : -EFAULT;
> }
>
> +/*
> + * Special Case: Efficient writev() support - only if all the IO buffers
> + * in the vector are page-aligned and IO sizes are page-multiple
> + * + total IO size is < wsize. We can map all of the vectors together
> + * and submit them in a single IO instead of operating on single entry
> + * in the vector.
> + */
> +static ssize_t nfs_direct_write_schedule_single_io(struct nfs_direct_req *dreq,
> + const struct iovec *iov,
> + unsigned long nr_segs,
> + int pages,
> + loff_t pos, int sync)
> +{
> + struct nfs_open_context *ctx = dreq->ctx;
> + struct inode *inode = ctx->path.dentry->d_inode;
> + unsigned long user_addr = (unsigned long)iov->iov_base;
> + size_t bytes = pages << PAGE_SHIFT;
> + struct rpc_task *task;
> + struct rpc_message msg = {
> + .rpc_cred = ctx->cred,
> + };
> + struct rpc_task_setup task_setup_data = {
> + .rpc_client = NFS_CLIENT(inode),
> + .rpc_message = &msg,
> + .callback_ops = &nfs_write_direct_ops,
> + .workqueue = nfsiod_workqueue,
> + .flags = RPC_TASK_ASYNC,
> + };
> + unsigned int pgbase;
> + int result;
> + int seg;
> + int mapped = 0;
> + int nr_pages;
> + ssize_t started = 0;
> + struct nfs_write_data *data;
> +
> + result = -ENOMEM;
> + data = nfs_writedata_alloc(pages);
> + if (unlikely(!data))
> + return result;
> +
> + pgbase = user_addr & ~PAGE_MASK;
> +
> + for (seg = 0; seg < nr_segs; seg++) {
> + user_addr = (unsigned long)iov[seg].iov_base;
> + nr_pages = iov[seg].iov_len >> PAGE_SHIFT;
> +
> + down_read(¤t->mm->mmap_sem);
> + result = get_user_pages(current, current->mm, user_addr,
> + nr_pages, 0, 0, &data->pagevec[mapped], NULL);
> + up_read(¤t->mm->mmap_sem);
> + if (result < 0) {
> + /* unmap what is done so far */
> + nfs_direct_release_pages(data->pagevec, mapped);
> + nfs_writedata_free(data);
> + return result;
> + }
> + mapped += result;
> + }
> +
> + get_dreq(dreq);
> + list_move_tail(&data->pages, &dreq->rewrite_list);
> +
> + data->req = (struct nfs_page *) dreq;
> + data->inode = inode;
> + data->cred = msg.rpc_cred;
> + data->args.fh = NFS_FH(inode);
> + data->args.context = ctx;
> + data->args.offset = pos;
> + data->args.pgbase = pgbase;
> + data->args.pages = data->pagevec;
> + data->args.count = bytes;
> + data->args.stable = sync;
> + data->res.fattr = &data->fattr;
> + data->res.count = bytes;
> + data->res.verf = &data->verf;
> + nfs_fattr_init(&data->fattr);
> +
> + task_setup_data.task = &data->task;
> + task_setup_data.callback_data = data;
> + msg.rpc_argp = &data->args;
> + msg.rpc_resp = &data->res;
> + NFS_PROTO(inode)->write_setup(data, &msg);
> +
> + task = rpc_run_task(&task_setup_data);
> + if (IS_ERR(task))
> + goto out;
> + rpc_put_task(task);
> +
> + started += bytes;
> +out:
> + if (started)
> + return started;
> + return result < 0 ? (ssize_t) result : -EFAULT;
> +}
> +
> static ssize_t nfs_direct_write_schedule_iovec(struct nfs_direct_req *dreq,
> const struct iovec *iov,
> unsigned long nr_segs,
> @@ -832,6 +1080,16 @@ static ssize_t nfs_direct_write_schedule
>
> get_dreq(dreq);
>
> + result = nfs_direct_check_single_io(dreq, iov, nr_segs, 1);
> + if (result) {
> + result = nfs_direct_write_schedule_single_io(dreq, iov, nr_segs,
> + result, pos, sync);
> + if (result < 0)
> + goto out;
> + requested_bytes += result;
> + pos += result;
> + goto out;
> + }
> for (seg = 0; seg < nr_segs; seg++) {
> const struct iovec *vec = &iov[seg];
> result = nfs_direct_write_schedule_segment(dreq, vec,
> @@ -853,6 +1111,7 @@ static ssize_t nfs_direct_write_schedule
> return result < 0 ? result : -EIO;
> }
>
> +out:
> if (put_dreq(dreq))
> nfs_direct_write_complete(dreq, dreq->inode);
> return 0;
>
>
> --
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--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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