> Why does the send operation block It has to, at least for TCP. The data resides in a user provided buffered .. that is why it is unbuffered. In TCP, it may need to retransmit if the data is not ACKed. Hence the user buffer must stay intact until the ACK is received.
The fully buffered logic does not have to wait for the ACK but can return as soon as the data is copied out of the user buffer. On Thu, Oct 14, 2021 at 12:54 PM Alexander Lunev <alexanderlu...@mail.ru.invalid> wrote: > Hi Gregory, > > > In the unbuffered send case, the send operation blocks until the > > packet is > > sent and ACKed. Per RFC 1122, the peer may delay sending the ACK for > > up to > > 500 Msec. So the performance of the unbuffered send is abysmal when > > sending to an RFC 1122 client. > > Why does the send operation block? > As I can see in the source code of the unbuffered send operation, TCP > packets are sent w/o waiting for ACKing each sent packet. The packets > are sent from NuttX until the remote TCP receiver's window covers the > total size of not acknowledged sent data. > > > I think that the unbuffered send logic is generally very bad and I > > have > > suggested removing it in the past. The buffered send performance is > > much > > better. We decided to retain the unbuffered send only because it > > does use > > less memory since no write buffering is required. > > Currently I'm using "unbuffered" send mode as in my case it > surprisingly provides twice as high throughput as "buffered" one. > Though, I initially expected that "buffered" send mode should have > better performance compared to "unbuffered" one. > > BTW, could you please say where I could read that discussion about > removing / retaining the unbuffered send in more details? > > Best regards, > Alexander Lunev. > > > On Thu, 2021-10-14 at 16:49 +0800, Gregory Nutt wrote: > > For people that need some background, this Wikipedia article may be > > helpful: https://en.wikipedia.org/wiki/TCP_delayed_acknowledgment. > > The > > SPLIT packet change is intended to work around issues when the > > unbuffered > > send is used with a peer that supports RFC 1122. > > > > In the unbuffered send case, the send operation blocks until the > > packet is > > sent and ACKed. Per RFC 1122, the peer may delay sending the ACK for > > up to > > 500 Msec. So the performance of the unbuffered send is abysmal when > > sending to an RFC 1122 client. > > > > The split basically breaks each TCP send into two packets. RFC 1122 > > does > > *require* that an ACK be sent after two packets are received. If you > > are > > seeing different behavior, then those peers are not conforming to RFC > > 1122. Perhaps there is some other standard. > > > > I think that the unbuffered send logic is generally very bad and I > > have > > suggested removing it in the past. The buffered send performance is > > much > > better. We decided to retain the unbuffered send only because it > > does use > > less memory since no write buffering is required. > > > > I am not 100% sure of this, but I still think that it would be better > > to > > remove the unbuffered TCP send logic rather than remove the packet > > split > > logic. I don't see how removing the split makes the unbuffered send > > better. It does not fix anything and certainly breaks the RFC 1122 > > case. > > The unbuffered TCP send behavior is bad without the split and, > > according to > > your PR, it is also bad with the split. But my feeling is not strong > > so > > just take that as my 2 cents worth. > > > > Greg > > > > On Tue, Oct 12, 2021 at 8:10 PM Takashi Yamamoto > > <yamam...@midokura.com.invalid> wrote: > > > > > hi, > > > > > > i submitted a PR to remove NET_TCP_SPLIT. > > > https://github.com/apache/incubator-nuttx/pull/4660 > > > > > > i'm posting this here because a feature removal should be reviewed > > > by > > > wider audience. > > > especially, if you are relying on it, please speak up. > > > > > > > > >
