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 ?
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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