netdev - Re: [Intel-wired-lan] [RFC v2 net-next 01/10] net: Add a new socket option for a future transmit time.

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Date:   Thu, 1 Feb 2018 12:55:59 -0800
From:   Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>
To:     Miroslav Lichvar <mlichvar@...hat.com>
Cc:     Richard Cochran <richardcochran@...il.com>, netdev@...r.kernel.org,
        john.stultz@...aro.org, Richard Cochran <rcochran@...utronix.de>,
        jiri@...nulli.us, ivan.briano@...el.com, henrik@...tad.us,
        jhs@...atatu.com, levi.pearson@...man.com,
        intel-wired-lan@...ts.osuosl.org, xiyou.wangcong@...il.com,
        tglx@...utronix.de, anna-maria@...utronix.de
Subject: Re: [Intel-wired-lan] [RFC v2 net-next 01/10] net: Add a new socket
 option for a future transmit time.

Hi,


On 02/01/2018 01:27 AM, Miroslav Lichvar wrote:
> On Wed, Jan 31, 2018 at 04:49:36PM -0800, Jesus Sanchez-Palencia wrote:
>> On 01/18/2018 09:13 AM, Richard Cochran wrote:
>>> Right, the clockid_t should be passed in through the CMSG along with
>>> the time.
>>
>> While implementing this today it crossed my mind that why don't we have the
>> clockid_t set per socket (e.g. as an argument to SO_TXTIME) instead of per packet?
> 
> I suspect that might have an impact on the performance. Even if the
> application doesn't use sendmmsg(), it would possibly have to call
> setsockopt() before each sendmsg() to change the clockid_t, right?


Yes. On the other hand, for applications that will be using only 1 clockid_t,
keeping it per packet will also have an impact as we'll be copying the same
value from the cmsg cookie into sk_buffs over and over.


> 
> If clockid_t could be set per packet, a special value could be used
> to allow sending on interfaces that don't support it.
> 
>> The only use-case that we could think of that would be 'blocked' was using
>> sendmmsg() to send a packet to different interfaces with a single syscall, but
>> I'm not sure how common that is.
> 
> The SO_TXTIME option will make sendmmsg() useful in applications where
> it wasn't before. For instance, an NTP server will be able to batch
> multiple responses as their transmit timestamps can be set accurately
> in advance and it's no longer necessary to send the responses as soon
> as they are assembled.
> 
> I think it would be nice the sendmmsg() calls didn't have to be split
> by clockid_t.


OK, fair enough. I will keep it per-packet for now as initially agreed and we
can revisit this later if needed.


Thanks,
Jesus