Hi Arseniy,
I left some comments in the patches, and I'm adding something also here:
On Fri, Jun 03, 2022 at 05:27:56AM +0000, Arseniy Krasnov wrote:
INTRODUCTION
Hello, this is experimental implementation of virtio vsock zerocopy
receive. It was inspired by TCP zerocopy receive by Eric Dumazet. This API uses
same idea: call 'mmap()' on socket's descriptor, then every 'getsockopt()' will
fill provided vma area with pages of virtio RX buffers. After received data was
processed by user, pages must be freed by 'madvise()' call with MADV_DONTNEED
flag set(if user won't call 'madvise()', next 'getsockopt()' will
fail).
If it is not too time-consuming, can we have a table/list to compare this
and the TCP zerocopy?
DETAILS
Here is how mapping with mapped pages looks exactly: first page mapping
contains array of trimmed virtio vsock packet headers (in contains only length
of data on the corresponding page and 'flags' field):
struct virtio_vsock_usr_hdr {
uint32_t length;
uint32_t flags;
uint32_t copy_len;
};
Field 'length' allows user to know exact size of payload within each sequence
of pages and 'flags' allows user to handle SOCK_SEQPACKET flags(such as message
bounds or record bounds). Field 'copy_len' is described below in 'v1->v2' part.
All other pages are data pages from RX queue.
Page 0 Page 1 Page N
[ hdr1 .. hdrN ][ data ] .. [ data ]
| | ^ ^
| | | |
| *-------------------*
| |
| |
*----------------*
Of course, single header could represent array of pages (when packet's
buffer is bigger than one page).So here is example of detailed mapping layout
for some set of packages. Lets consider that we have the following sequence of
packages: 56 bytes, 4096 bytes and 8200 bytes. All pages: 0,1,2,3,4 and 5 will
be inserted to user's vma(vma is large enough).
In order to have a "userspace polling-friendly approach" and reduce
number of syscall, can we allow for example the userspace to mmap at
least the first header before packets arrive.
Then the userspace can poll a flag or other fields in the header to
understand that there are new packets.
That would be cool, but in the meantime it would be nice to behave
similarly to TCP, which is why the comparison table I mentioned earlier
would be useful.
Page 0: [[ hdr0 ][ hdr 1 ][ hdr 2 ][ hdr 3 ] ... ]
Page 1: [ 56 ]
Page 2: [ 4096 ]
Page 3: [ 4096 ]
Page 4: [ 4096 ]
Page 5: [ 8 ]
Page 0 contains only array of headers:
'hdr0' has 56 in length field.
'hdr1' has 4096 in length field.
'hdr2' has 8200 in length field.
'hdr3' has 0 in length field(this is end of data marker).
Page 1 corresponds to 'hdr0' and has only 56 bytes of data.
Page 2 corresponds to 'hdr1' and filled with data.
Page 3 corresponds to 'hdr2' and filled with data.
Page 4 corresponds to 'hdr2' and filled with data.
Page 5 corresponds to 'hdr2' and has only 8 bytes of data.
This patchset also changes packets allocation way: today implementation
uses only 'kmalloc()' to create data buffer. Problem happens when we try to map
such buffers to user's vma - kernel forbids to map slab pages to user's vma(as
pages of "not large" 'kmalloc()' allocations are marked with PageSlab flag and
"not large" could be bigger than one page). So to avoid this, data buffers now
allocated using 'alloc_pages()' call.
TESTS
This patchset updates 'vsock_test' utility: two tests for new feature
were added. First test covers invalid cases. Second checks valid transmission
case.
BENCHMARKING
For benchmakring I've added small utility 'rx_zerocopy'. It works in
client/server mode. When client connects to server, server starts sending exact
amount of data to client(amount is set as input argument).Client reads data and
waits for next portion of it. Client works in two modes: copy and zero-copy. In
copy mode client uses 'read()' call while in zerocopy mode sequence of 'mmap()'
/'getsockopt()'/'madvise()' are used. Smaller amount of time for transmission
is better. For server, we can set size of tx buffer and for client we can set
size of rx buffer or rx mapping size(in zerocopy mode). Usage of this utility
is quiet simple:
For client mode:
./rx_zerocopy --mode client [--zerocopy] [--rx]
For server mode:
./rx_zerocopy --mode server [--mb] [--tx]
[--mb] sets number of megabytes to transfer.
[--rx] sets size of receive buffer/mapping in pages.
[--tx] sets size of transmit buffer in pages.
I checked for transmission of 4000mb of data. Here are some results:
size of rx/tx buffers in pages
*---------------------------------------------------*
| 8 | 32 | 64 | 256 | 512 |
*--------------*--------*----------*---------*----------*----------*
| zerocopy | 24 | 10.6 | 12.2 | 23.6 | 21 | secs to
*--------------*---------------------------------------------------- process
| non-zerocopy | 13 | 16.4 | 24.7 | 27.2 | 23.9 | 4000 mb
*--------------*----------------------------------------------------
Result in first column(where non-zerocopy works better than zerocopy) happens
because time, spent in 'read()' system call is smaller that time in 'getsockopt'
+ 'madvise'. I've checked that.
I think, that results are not so impressive, but at least it is not worse than
copy mode and there is no need to allocate memory for processing date.
PROBLEMS
Updated packet's allocation logic creates some problem: when host gets
data from guest(in vhost-vsock), it allocates at least one page for each packet
(even if packet has 1 byte payload). I think this could be resolved in several
ways:
1) Make zerocopy rx mode disabled by default, so if user didn't enable
it, current 'kmalloc()' way will be used. <<<<<<< (IMPLEMENTED IN V2)
Yep, but I think we should not allow to change it while we are connected
(see comments in the patches.)
2) Use 'kmalloc()' for "small" packets, else call page allocator. But
in this case, we have mix of packets, allocated in two different ways thus
during zerocopying to user(e.g. mapping pages to vma), such small packets will
be handled in some stupid way: we need to allocate one page for user, copy data
to it and then insert page to user's vma.
v1 -> v2:
1) Zerocopy receive mode could be enabled/disabled(disabled by default). I
didn't use generic SO_ZEROCOPY flag, because in virtio-vsock case this
feature depends on transport support. Instead of SO_ZEROCOPY, AF_VSOCK
layer flag was added: SO_VM_SOCKETS_ZEROCOPY, while previous meaning of
SO_VM_SOCKETS_ZEROCOPY(insert receive buffers to user's vm area) now
renamed to SO_VM_SOCKETS_MAP_RX.
2) Packet header which is exported to user now get new field: 'copy_len'.
This field handles special case: user reads data from socket in non
zerocopy way(with disabled zerocopy) and then enables zerocopy feature.
In this case vhost part will switch data buffer allocation logic from
'kmalloc()' to direct calls for buddy allocator. But, there could be
some pending 'kmalloc()' allocated packets in socket's rx list, and then
user tries to read such packets in zerocopy way, dequeue will fail,
because SLAB pages could not be inserted to user's vm area. So when such
packet is found during zerocopy dequeue, dequeue loop will break and
'copy_len' will show size of such "bad" packet. After user detects this
case, it must use 'read()/recv()' calls to dequeue such packet.
3) Also may be move this features under config option?
Do you mean a build config like CONFIG_VSOCK_ZERO_COPY?
I'm not sure it's needed.
Thanks,
Stefano
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