Futures are great for complicated asynchronous operations, but for a
synchronous operation like destroying a packet after transmit, or
converting a buffer to a packet during receive, they're overkill.
This patchset fixes those two cases in virtio, in which futures
are used as an abstraction layer between vring and the transmit/receive
queues, by converting vring into a template, so that the completion function
can be adjusted for the transmit or receive case during compile time instead
of at run time.
10% improvement on httpd with --smp 1, >20% with --smp 3.
Move completion handling (destroy packet, adjust descriptors count) to
a completion function rather than a future. Reduces allocations and task
executed.
Move completion handling (destroy packet, adjust descriptors count) to
a completion function rather than a future. Reduces allocations and task
executed.
Currently vring request completions are handled by fulfilling a promise
contained in the request. While promises are very flexible, this comes
at a cost (allocating and executing a task), and this flexibility is unneeded
when request handling is very regular (such as in virtio-net rx and tx
completion handling).
Make vring more flexible by allowing the completion function to be specified
as a template parameter. No changes to the actual users - they now specify
the completion function as fulfilling the same promise as vring previously
did.
wait_and_process() expects an std::function<>, but we pass it a lambda,
forcing it to allocate.
Prepare the sdt::function<> in advance, so it can pass by reference.
Since we control the capacity, we can force it to be a power of two,
and use masking instead of tests to handle wraparound.
A side benefit is that we don't have to allocate an extra element.
We're currently using boost::lockfree::consume_all() to consume
smp requests, but this has two problems:
1. consume_all() calls consume_one() internally, which means it accesses
the ring index once per message
2 we interleave calling the request function with accessing the ring, which
allows the other side to access the ring again, bouncing ring cache lines.
Fix by copying all available items in one show, using pop(array), and then
processing them afterwards.
We're currently using boost::lockfree::consume_all() to consume
smp completions, but this has two problems:
1. consume_all() calls consume_one() internally, which means it accesses
the ring index once per message
2 we interleave calling the request function with accessing the ring, which
allows the other side to access the ring again, bouncing ring cache lines.
Fix by copying all available items in one show, using pop(array), and then
processing them afterwards.
Instead of incurring the overhead of pushing a message down the queue (two
cache line misses), amortize of over 16 messages (3/4 cache line misses per
batch).
Batch size is limited by poll frequency, so we should adjust that
dynamically.
If it needs to be resized, it will cause a deallocation on the wrong cpu,
so initialize it on the sending cpu.
Does not break with circular_buffer<>, but it's not going to be a
circular_buffer<> for long.
Instead of placing packets directly into the virtio ring, add them to
a temporary queue, and flush it when we are polled. This reduces
cross-cpu writes and kicks.
This is a little tricky, since we only know we want hugetlbfs after memory
has been initialized, so we start up in anonymous memory, and later
switch to hugetlbfs by copying it to hugetlb-backed memory and mremap()ing
it back into place.
This patch uses the NIC's capability to calculate in hardware the IP, TCP
and UDP checksums on outgoing packets, instead of us doing this on the
sending CPU. This can save us quite a bit of calculations (especially for
the TCP/UDP checksum of full-sized packets), and avoid cache-polution on
the CPU when sending cold data.
On my setup this patch improves the performance of a single-cpu memcached
by 6%. Together with the recent patch for receive-side checksum offloading,
the total improvement is 10%.
This patch is somewhat complicated by the fact we have so many different
combinations of checksum-offloading capabilities; While virtio can only
offload layer-4 checksumming (tcp/udp), dpdk lets us offload both ip and
layer-4 checksum. Moreover, some packets are just IP but not TCP/UDP
(e.g., ICMP), and some packets are not even IP (e.g., ARP), so this
patch modifies a few of the hardware-features flags and the per-packet
offload-information flags to fit our new needs.
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Since we switched temporary_buffer to malloc(), it now longer throws
an exception after running out of memory, which leads to a segfault
when referencing a null buffer.
