netdev - Re: [PATCH net-next 0/6] net: make TCP preemptible

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Date:	Thu, 28 Apr 2016 10:41:10 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	Eric Dumazet <edumazet@...gle.com>,
	"David S . Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 0/6] net: make TCP preemptible

On Thu, 2016-04-28 at 10:23 -0700, Alexei Starovoitov wrote:
> On Wed, Apr 27, 2016 at 10:25:46PM -0700, Eric Dumazet wrote:
> > Most of TCP stack assumed it was running from BH handler.
> > 
> > This is great for most things, as TCP behavior is very sensitive
> > to scheduling artifacts.
> > 
> > However, the prequeue and backlog processing are problematic,
> > as they need to be flushed with BH being blocked.
> > 
> > To cope with modern needs, TCP sockets have big sk_rcvbuf values,
> > in the order of 16 MB.
> > This means that backlog can hold thousands of packets, and things
> > like TCP coalescing or collapsing on this amount of packets can
> > lead to insane latency spikes, since BH are blocked for too long.
> > 
> > It is time to make UDP/TCP stacks preemptible.
> > 
> > Note that fast path still runs from BH handler.
> 
> this looks pretty awesome.

Yes, I am pretty excited ;)

> the change will make the backlog run in bh enabled, so that one
> large flow reciever will not penalize the rest of the system, right?

Not only large flows, but flows with losses/reorders.
Typically many flows are in this case when a congestion collapse
happens.



> but you're saying that prequeue is also expensive, but not touched
> by this patchset? was it addressed by your eariler patch?
> Or more work still tbd?

prequeue is handled by "[2/6] tcp: do not block bh during prequeue
processing"

Note that I also sent a patch earlier (("tcp: give prequeue mode some
care")) to control max size of prequeue to 32 packets.

> I'm just trying to understand more about tcp stack.
> 
Sure ;)

I also have a patch to add scheduling point in sendmsg() (ie : draining
the backlog if not empty) for each new skb added to the write queue.

Since each skb is about 64KB (with GSO/TSO), it means an application no
longer will hold the socket lock too long, even when doing a
write()/sendmsg() of say 8 MB at once ;)