netdev - Re: [PATCH net-next] doc: document MSG

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Date:   Fri, 1 Sep 2017 11:47:06 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] doc: document MSG_ZEROCOPY

On Thu, Aug 31, 2017 at 11:10 PM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Thu, Aug 31, 2017 at 11:04:41PM -0400, Willem de Bruijn wrote:
>> On Thu, Aug 31, 2017 at 10:10 PM, Alexei Starovoitov
>> <alexei.starovoitov@...il.com> wrote:
>> > On Thu, Aug 31, 2017 at 05:00:13PM -0400, Willem de Bruijn wrote:
>> >> From: Willem de Bruijn <willemb@...gle.com>
>> >>
>> >> Documentation for this feature was missing from the patchset.
>> >> Copied a lot from the netdev 2.1 paper, addressing some small
>> >> interface changes since then.
>> >>
>> >> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>> > ...
>> >> +Notification Batching
>> >> +~~~~~~~~~~~~~~~~~~~~~
>> >> +
>> >> +Multiple outstanding packets can be read at once using the recvmmsg
>> >> +call. This is often not needed. In each message the kernel returns not
>> >> +a single value, but a range. It coalesces consecutive notifications
>> >> +while one is outstanding for reception on the error queue.
>> >> +
>> >> +When a new notification is about to be queued, it checks whether the
>> >> +new value extends the range of the notification at the tail of the
>> >> +queue. If so, it drops the new notification packet and instead increases
>> >> +the range upper value of the outstanding notification.
>> >
>> > Would it make sense to mention that max notification range is 32-bit?
>> > So each 4Gbyte of xmit bytes there will be a notification.
>> > In modern 40Gbps NICs it's not a lot. Means that there will be
>> > at least one notification every second.
>> > Or I misread the code?
>>
>> You're right. The doc does mention that the counter and range
>> are 32-bit. I can state more explicitly that that bounds the working
>> set size to 4GB. Do you expect this to be problematic? Processing
>> a single notification per 4GB of data should not be a significant
>> cost in itself.

Actually, the counter is not a byte counter. It is incremented on each
system call that sends data with MSG_ZEROCOPY. So the 4GB limit
would only hold if a caller sends single byte requests at a time.

I will make this more clear in v2.

>
> I think 4GB is fine. Just there was an idea that in cases when
> notification of transmission can be known by other means the user space
> could have skipped reading errqeuee completely, but looks like it
> still needs to poll. That's fine.
>> > Thanks for the doc!
>>
>> Thanks for reviewing :)
>>
>> >
>> > Acked-by: Alexei Starovoitov <ast@...nel.org>
>> >