sendfile(2) SF_NOPUSH flag proposal

[Peter Jeremy]

On Tue, May 27, 2003 at 02:49:35PM +0400, Igor Sysoev wrote:
>Actually 6 vs. 6 for this 8K file. But I said about another thing.
>Let's see 48K file and 250 bytes header. sendfile() usually sends
>it as 4K or 8K hunks so there are 48/8 * 6 + 1 (header) = 37 packets.
>But (48K + 250) / 1460 = 33 * 1460 + 1270 i.e. 34 packets.
>It's 8% decrease of data packets. Add here the possible retransmitions.

Why is the number of data packets so important?  If you repeat the
calculation considering bytes across the wire (assuming Ethernet),
then the saving is closer to 0.4% - this is in the noise.

For that matter, have you considered the impact of Path MTU discovery?

I think possible retransmissions are irrelevant here.  If your packet
loss is anything above negligible then you have other problems.  If
the retransmission is caused by transmission noise, then the smaller
packets are less likely to get hit.  And the sender is likely to
retransmit a full packet rather than the small packet originally sent.

>> Really: it's in the noise.  Unless you are paying by packet
>> count, you probably shouldn't care.
>
>So do you consider that IP fragmentation is the good thing ?

"IP fragmentation" normally refers to a single IP packet being
split up into multiple smaller packets by a router.  It has nothing
to do with the topic under discussion.  If anything, transmitting
smaller IP packets reduces the likelihood that an intervening router
will need to fragment packets - so your patch actually increases
the probability of IP fragmentation.

Peter