On 2/26/24 14:27, Jens Axboe wrote:
On 2/26/24 7:02 AM, Dylan Yudaken wrote:
On Mon, Feb 26, 2024 at 1:38?PM Jens Axboe <axboe@xxxxxxxxx> wrote:
On 2/26/24 3:47 AM, Dylan Yudaken wrote:
On Sun, Feb 25, 2024 at 12:46?AM Jens Axboe <axboe@xxxxxxxxx> wrote:
This works very much like the receive side, except for sends. The idea
is that an application can fill outgoing buffers in a provided buffer
group, and then arm a single send that will service them all. For now
this variant just terminates when we are out of buffers to send, and
hence the application needs to re-arm it if IORING_CQE_F_MORE isn't
set, as per usual for multishot requests.
This feels to me a lot like just using OP_SEND with MSG_WAITALL as
described, unless I'm missing something?
How so? MSG_WAITALL is "send X amount of data, and if it's a short send,
try again" where multishot is "send data from this buffer group, and
keep sending data until it's empty". Hence it's the mirror of multishot
on the receive side. Unless I'm misunderstanding you somehow, not sure
it'd be smart to add special meaning to MSG_WAITALL with provided
buffers.
_If_ you have the data upfront these are very similar, and only differ
in that the multishot approach will give you more granular progress
updates. My point was that this might not be a valuable API to people
for only this use case.
Not sure I agree, it feels like attributing a different meaning to
MSG_WAITALL if you use a provided buffer vs if you don't. And that to me
would seem to be confusing. Particularly when we have multishot on the
receive side, and this is identical, just for sends. Receives will keep
receiving as long as there are buffers in the provided group to receive
into, and sends will keep sending for the same condition. Either one
will terminate if we run out of buffers.
If you make MSG_WAITALL be that for provided buffers + send, then that
behaves differently than MSG_WAITALL with receive, and MSG_WAITALL with
send _without_ provided buffers. I don't think overloading an existing
flag for this purposes is a good idea, particularly when we already have
the existing semantics for multishot on the receive side.
I'm actually with Dylan on that and wonder where the perf win
could come from. Let's assume TCP, sends are usually completed
in the same syscall, otherwise your pacing is just bad. Thrift,
for example, collects sends and packs into one multi iov request
during a loop iteration. If the req completes immediately then
the userspace just wouldn't have time to push more buffers by
definition (assuming single threading).
If you actually need to poll tx, you send a request and collect
data into iov in userspace in background. When the request
completes you send all that in batch. You can probably find
a niche example when batch=1 in this case, but I don't think
anyone would care.
The example doesn't use multi-iov, and also still has to
serialise requests, which naturally serialises buffer consumption
w/o provided bufs.
You do make a good point about MSG_WAITALL though - multishot send
doesn't really make sense to me without MSG_WAITALL semantics. I
cannot imagine a useful use case where the first buffer being
partially sent will still want the second buffer sent.
Right, and I need to tweak that. Maybe we require MSG_WAITALL, or we
make it implied for multishot send. Currently the code doesn't deal with
that.
Maybe if MSG_WAITALL isn't set and we get a short send we don't set
CQE_F_MORE and we just stop. If it is set, then we go through the usual
retry logic. That would make it identical to MSG_WAITALL send without
multishot, which again is something I like in that we don't have
different behaviors depending on which mode we are using.
I actually could imagine it being useful for the previous patches' use
case of queuing up sends and keeping ordering,
and I think the API is more obvious (rather than the second CQE
sending the first CQE's data). So maybe it's worth only
keeping one approach?
And here you totally lost me :-)
I am suggesting here that you don't really need to support buffer
lists on send without multishot.
That is certainly true, but I also don't see a reason _not_ to support
it. Again mostly because this is how receive and everything else works.
The app is free to issue a single SQE for send without multishot, and
pick the first buffer and send it.
Multishot sound interesting, but I don't see it much useful if
you terminate when there are no buffers. Otherwise, if it continues
to sit in, someone would have to wake it up
It's a slightly confusing API (to me) that you queue PushBuffer(A),
Send(A), PushBuffer(B), Send(B)
and get back Res(B), Res(A) which are in fact in order A->B.
Now I'm confused again. If you do do the above sequence, the first CQE
posted would be Res(A), and then Res(B)?
Instead you could queue up PushBuffer(A), Send(Multishot),
PushBuffer(B), and get back Res(Multishot), Res(Multishot)
which are in order A -> B.
There should be no difference in ordering of the posted completion
between the two.
--
Pavel Begunkov
