Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

Ingo Molnar
2007-02-26T13:20:06Z

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> > My tests show that with 4k connections per second (8k concurrency)
> > more than 20k connections of 80k total block in tcp_sendmsg() over
> > gigabit lan between quite fast machines.
>
> Why do people *keep* taking this up as an issue?
>
> Use select/poll/epoll/kevent/whatever for event mechanisms. STOP
> CLAIMING that you'd use threadlets/syslets/aio for that. It's been
> pointed out over and over and over again, and yet you continue to make
> the same mistake, Evgeniy.
>
> So please read that sentence ten times, and then don't continue to
> make that same mistake. PLEASE.
>
> Event mechanisms are *superior* for events. But they *suck* for things
> that aren't events, but are actual code execution with random places
> that can block. THE TWO THINGS ARE TOTALLY AND UTTERLY INDEPENDENT!

Note that even for something tasks are supposed to suck at, and even if
used in extremely stupid ways, they perform reasonably well in practice
;-)

And i fully agree: specialization based on knowledge about frequency of
blocking will always be useful - if not /forced/ on the workflow
architecture and if not overdone. On the other hand, fully event-driven
servers based on 'nonblocking' calls, which Evgeniy is advocating and
which the kevent model is forcing upon userspace, is pure madness.

We very much can and should use things like epoll for events that we
expect to happen asynchronously 100% of the time - it just makes no
sense for those events to take up 4-5K of RAM apiece, when they could
also be only using up the 32 bytes that say a pending timer takes. I've
posted the code for that, how to do an 'outer' epoll loop around an
internal threadlep iterator. But those will always be very narrow event
sources, and likely wont (and shouldnt) cover 'request-internal'
processing.

but otherwise, there is no real difference between a task that is
scheduled and a request that is queued, 'other' than the size of the
request (the task takes 4-5K of RAM), and the register context (64-128
bytes on most CPUs, the loading of which is optimized to death).

Which difference can still be significant for certain workloads, so we
certainly dont want to prohibit specialized event interfaces and force
generic threads on everything. But for anything that isnt a raw and
natural external event source (time, network, disk, user-generated)
there shouldnt be much of an event queueing abstraction i believe (other
than what we get 'for free' within epoll, from having poll()-able files)
- and even for those event sources threadlets offer a pretty good run
for the money.

one can always find the point and workload where say 40,000 threads
start trashing the L2 cache, but where 40,000 queued special requests
are still fully in cache, and produce spectacular numbers.

Ingo