Subject: Re: [PATCH RFC net-next 1/6] sock: MSG_PEEK support for sk_error_queue

From: Sowmini Varadhan <[email-masked]>
Date: Thu, 18 Jan 2018 06:02:07 -0500

On (01/17/18 18:50), Willem de Bruijn wrote:
> 
> This can cause reordering with parallel readers. Can we avoid the need
> for peeking? It also caused a slew of subtle bugs previously.

Yes, I did notice the potential for re-ordering when writing the patch..
but these are not actuallly messages from the wire, so is re-ordering 
fatal?

In general, I"m not particularly attached to this solution- in my
testing, I'm seeing that it's possible to reduce the latency  and still
take a hit on the throughput if the application does not reap the
completion notifciation (and send out new data) efficiently

Some (radically differnt) alternatives that were suggested to me 

- send up all the cookies as ancillary data with recvmsg (i.e., send
  it as a cmsgdata along with actual data from the wire). In most
  cases, the application has data to read, anyway. If it doesnt (pure
  sender), we could wake up recvmsg with 0 bytes of data, but with
  the cookie info in the ancillary data. This feels not-so-elegant
  to me, but I suppose it would have the benefit of optimizing on
  the syscall overhead.. (and you could use MSG_CTRUNC to handle
  the case of insuufficient bufffer for cookies, sending the rest
  on the next call)..
- allow application to use a setsockopt on the rds socket, with
  some shmem region, into which the kernel could write the cookies,
  Let application reap cookies without syscall overhead from that
  shmem region..

> How about just define a max number of cookies and require the caller
> to always read with sufficient room to hold them?

This may be "good enough" as well, maybe allow a max of (say) 16 cookies,
and set up  the skb's in the error queue to send up batches of 16 cookies
at a time?

--Sowmini