On Sat, Jun 03, 2023 at 11:49:22PM +0300, Arseniy Krasnov wrote:
Hello,
DESCRIPTION
this is MSG_ZEROCOPY feature support for virtio/vsock. I tried to follow
current implementation for TCP as much as possible:
1) Sender must enable SO_ZEROCOPY flag to use this feature. Without this
flag, data will be sent in "classic" copy manner and MSG_ZEROCOPY
flag will be ignored (e.g. without completion).
2) Kernel uses completions from socket's error queue. Single completion
for single tx syscall (or it can merge several completions to single
one). I used already implemented logic for MSG_ZEROCOPY support:
'msg_zerocopy_realloc()' etc.
Difference with copy way is not significant. During packet allocation,
non-linear skb is created and filled with pinned user pages.
There are also some updates for vhost and guest parts of transport - in
both cases i've added handling of non-linear skb for virtio part. vhost
copies data from such skb to the guest's rx virtio buffers. In the guest,
virtio transport fills tx virtio queue with pages from skb.
Head of this patchset is:
https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=d20dd0ea14072e8a90ff864b2c1603bd68920b4b
This version has several limits/problems:
1) As this feature totally depends on transport, there is no way (or it
is difficult) to check whether transport is able to handle it or not
during SO_ZEROCOPY setting. Seems I need to call AF_VSOCK specific
setsockopt callback from setsockopt callback for SOL_SOCKET, but this
leads to lock problem, because both AF_VSOCK and SOL_SOCKET callback
are not considered to be called from each other. So in current version
SO_ZEROCOPY is set successfully to any type (e.g. transport) of
AF_VSOCK socket, but if transport does not support MSG_ZEROCOPY,
tx routine will fail with EOPNOTSUPP.
^^^
This is still no resolved :(
2) When MSG_ZEROCOPY is used, for each tx system call we need to enqueue
one completion. In each completion there is flag which shows how tx
was performed: zerocopy or copy. This leads that whole message must
be send in zerocopy or copy way - we can't send part of message with
copying and rest of message with zerocopy mode (or vice versa). Now,
we need to account vsock credit logic, e.g. we can't send whole data
once - only allowed number of bytes could sent at any moment. In case
of copying way there is no problem as in worst case we can send single
bytes, but zerocopy is more complex because smallest transmission
unit is single page. So if there is not enough space at peer's side
to send integer number of pages (at least one) - we will wait, thus
stalling tx side. To overcome this problem i've added simple rule -
zerocopy is possible only when there is enough space at another side
for whole message (to check, that current 'msghdr' was already used
in previous tx iterations i use 'iov_offset' field of it's iov iter).
^^^
Discussed as ok during v2. Link:
https://lore.kernel.org/netdev/23guh3txkghxpgcrcjx7h62qsoj3xgjhfzgtbmqp2slrz3rxr4@zya2z7kwt75l/
3) loopback transport is not supported, because it requires to implement
non-linear skb handling in dequeue logic (as we "send" fragged skb
and "receive" it from the same queue). I'm going to implement it in
next versions.
^^^ fixed in v2
4) Current implementation sets max length of packet to 64KB. IIUC this
is due to 'kmalloc()' allocated data buffers. I think, in case of
MSG_ZEROCOPY this value could be increased, because 'kmalloc()' is
not touched for data - user space pages are used as buffers. Also
this limit trims every message which is > 64KB, thus such messages
will be send in copy mode due to 'iov_offset' check in 2).
^^^ fixed in v2
PATCHSET STRUCTURE
Patchset has the following structure:
1) Handle non-linear skbuff on receive in virtio/vhost.
2) Handle non-linear skbuff on send in virtio/vhost.
3) Updates for AF_VSOCK.
4) Enable MSG_ZEROCOPY support on transports.
5) Tests/tools/docs updates.
PERFORMANCE
Performance: it is a little bit tricky to compare performance between
copy and zerocopy transmissions. In zerocopy way we need to wait when
user buffers will be released by kernel, so it is like synchronous
path (wait until device driver will process it), while in copy way we
can feed data to kernel as many as we want, don't care about device
driver. So I compared only time which we spend in the 'send()' syscall.
Then if this value will be combined with total number of transmitted
bytes, we can get Gbit/s parameter. Also to avoid tx stalls due to not
enough credit, receiver allocates same amount of space as sender needs.
Sender:
./vsock_perf --sender <CID> --buf-size <buf size> --bytes 256M [--zc]
Receiver:
./vsock_perf --vsk-size 256M
I run tests on two setups: desktop with Core i7 - I use this PC for
development and in this case guest is nested guest, and host is normal
guest. Another hardware is some embedded board with Atom - here I don't
have nested virtualization - host runs on hw, and guest is normal guest.
G2H transmission (values are Gbit/s):
Core i7 with nested guest. Atom with normal guest.
*-------------------------------* *-------------------------------*
| | | | | | | |
| buf size | copy | zerocopy | | buf size | copy | zerocopy |
| | | | | | | |
*-------------------------------* *-------------------------------*
| 4KB | 3 | 10 | | 4KB | 0.8 | 1.9 |
*-------------------------------* *-------------------------------*
| 32KB | 20 | 61 | | 32KB | 6.8 | 20.2 |
*-------------------------------* *-------------------------------*
| 256KB | 33 | 244 | | 256KB | 7.8 | 55 |
*-------------------------------* *-------------------------------*
| 1M | 30 | 373 | | 1M | 7 | 95 |
*-------------------------------* *-------------------------------*
| 8M | 22 | 475 | | 8M | 7 | 114 |
*-------------------------------* *-------------------------------*
H2G:
Core i7 with nested guest. Atom with normal guest.
*-------------------------------* *-------------------------------*
| | | | | | | |
| buf size | copy | zerocopy | | buf size | copy | zerocopy |
| | | | | | | |
*-------------------------------* *-------------------------------*
| 4KB | 20 | 10 | | 4KB | 4.37 | 3 |
*-------------------------------* *-------------------------------*
| 32KB | 37 | 75 | | 32KB | 11 | 18 |
*-------------------------------* *-------------------------------*
| 256KB | 44 | 299 | | 256KB | 11 | 62 |
*-------------------------------* *-------------------------------*
| 1M | 28 | 335 | | 1M | 9 | 77 |
*-------------------------------* *-------------------------------*
| 8M | 27 | 417 | | 8M | 9.35 | 115 |
*-------------------------------* *-------------------------------*
* Let's look to the first line of both tables - where copy is better
than zerocopy. I analyzed this case more deeply and found that
bottleneck is function 'vhost_work_queue()'. With 4K buffer size,
caller spends too much time in it with zerocopy mode (comparing to
copy mode). This happens only with 4K buffer size. This function just
calls 'wake_up_process()' and its internal logic does not depends on
skb, so i think potential reason (may be) is interval between two
calls of this function (e.g. how often it is called). Note, that
'vhost_work_queue()' differs from the same function at guest's side of
transport: 'virtio_transport_send_pkt()' uses 'queue_work()' which
i think is more optimized for worker purposes, than direct call to
'wake_up_process()'. But again - this is just my assumption.
Thanks for the analysis, however for small payloads it makes sense that
the cost might be too high that optimization does not bring benefits.
Loopback:
Core i7 with nested guest. Atom with normal guest.
*-------------------------------* *-------------------------------*
| | | | | | | |
| buf size | copy | zerocopy | | buf size | copy | zerocopy |
| | | | | | | |
*-------------------------------* *-------------------------------*
| 4KB | 8 | 7 | | 4KB | 1.8 | 1.3 |
*-------------------------------* *-------------------------------*
| 32KB | 38 | 44 | | 32KB | 10 | 10 |
*-------------------------------* *-------------------------------*
| 256KB | 55 | 168 | | 256KB | 15 | 36 |
*-------------------------------* *-------------------------------*
| 1M | 53 | 250 | | 1M | 12 | 45 |
*-------------------------------* *-------------------------------*
| 8M | 40 | 344 | | 8M | 11 | 74 |
*-------------------------------* *-------------------------------*
I analyzed performace difference more deeply for the following setup:
server: ./vsock_perf --vsk-size 16M
client: ./vsock_perf --sender 2 --bytes 16M --buf-size 16K/4K [--zc]
In other words I send 16M of data from guest to host in copy/zerocopy
modes and with two different sizes of buffer - 4K and 64K. Let's see
to tx path for both modes - it consists of two steps:
copy:
1) Allocate skb of buffer's length.
2) Copy data to skb from buffer.
zerocopy:
1) Allocate skb with header space only.
2) Pin pages of the buffer and insert them to skb.
I measured average number of ns (returned by 'ktime_get()') for each
step above:
1) Skb allocation (for both copy and zerocopy modes).
2) For copy mode in 'memcpy_to_msg()' - copying.
3) For zerocopy mode in '__zerocopy_sg_from_iter()' - pinning.
Here are results for copy mode:
*-------------------------------------*
| buf | skb alloc | 'memcpy_to_msg()' |
*-------------------------------------*
| | | |
| 64K | 5000ns | 25000ns |
| | | |
*-------------------------------------*
| | | |
| 4K | 800ns | 2200ns |
| | | |
*-------------------------------------*
Here are results for zerocopy mode:
*-----------------------------------------------*
| buf | skb alloc | '__zerocopy_sg_from_iter()' |
*-----------------------------------------------*
| | | |
| 64K | 250ns | 3500ns |
| | | |
*-----------------------------------------------*
| | | |
| 4K | 250ns | 3000ns |
| | | |
*-----------------------------------------------*
I guess that reason of zerocopy performance is low overhead for page
pinning: there is big difference between 4K and 64K in case of copying
(25000 vs 2200), but in pinning case - just 3000 vs 3500.
So, zerocopy is faster than classic copy mode, but of course it requires
specific architecture of application due to user pages pinning, buffer
size and alignment.
Makes sense!
NOTES
If host fails to send data with "Cannot allocate memory", check value
/proc/sys/net/core/optmem_max - it is accounted during completion skb
allocation. Try to update it to for example 1M and try send again:
"echo 1048576 > /proc/sys/net/core/optmem_max" (as root).
TESTING
This patchset includes set of tests for MSG_ZEROCOPY feature. I tried to
cover new code as much as possible so there are different cases for
MSG_ZEROCOPY transmissions: with disabled SO_ZEROCOPY and several io
vector types (different sizes, alignments, with unmapped pages). I also
run tests with loopback transport and run vsockmon. In v3 i've added
io_uring test as separated application.
LET'S SPLIT PATCHSET TO MAKE REVIEW EASIER
In v3 Stefano Garzarella <sgarzare@xxxxxxxxxx> asked to split this patchset
for several parts, because it looks too big for review. I think in this
version (v4) we can do it in the following way:
[0001 - 0005] - this is preparation for virtio/vhost part.
[0006 - 0009] - this is preparation for AF_VSOCK part.
[0010 - 0013] - these patches allows to trigger logic from the previous
two parts.
[0014 - rest] - updates for doc, tests, utils. This part doesn't touch
kernel code and looks not critical.
Yeah, I like this split, but I'd include 14 in the (10, 13) group.
I have reviewed most of them and I think we are well on our way :-)
I've already seen that Bobby suggested changes for v5, so I'll review
that version better.
Great work so far!
Thanks,
Stefano
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