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

Ingo Molnar
2007-02-27T12:41:09Z

* Theodore Tso <tytso@mit.edu> wrote:

> I think what you are not hearing, and what everyone else is saying
> (INCLUDING Linus), is that for most programmers, state machines are
> much, much harder to program, understand, and debug compared to
> multi-threaded code. [...]

btw., another crutial thing that i think Evgeniy is missing is that
threadlets /enable/ event loops to be used in practice! Right now the
epoll/kevent programming model requires a total 100% avoidance of all
context-switching in the 'main' event handler context while handling a
request. If just 1% of all requests happen to block it might cause a
/complete/ breakdown of an event loop's performance - it can easily
cause a 10x drop in performance or worse!

So context-switching has to be avoided in 100% of the code that runs
while handling requests, file descriptors have to be set to nonblocking
(causing extra system calls), and all the syscalls that might return
incomplete with either -EINVAL or with a short read/write have to be
converted into a state machine. (or in the alternative, user-space
threading has to be used, which opens up another hornet's nest)

/That/ is the main inhibiting factor of the measured use of event loops
within Linux! It has zero integration capabilities with 'usual' coding
techniques - driving the costs of its application up in the sky, and
pushing event based servers into niches.

With threadlets the picture changes dramatically: all we have to
concentrate on to get the performance of "100% event based servers" is
to handle 'most' rescheduling events in the event loop. A 10-20% context
switching ratio does not hurt at all. (it causes ~1% of throughput
loss.)

Furthermore, even if a particular configuration or module of the server
(say Apache) happens to trigger a high rate of scheduling, the
performance breakdown model of threadlets is /vastly/ superior to event
based servers. The measurements so far have shown that the absolute
worst-case threading server performance is at around 60% of that of
non-context-switching servers - and even that level is reached
gradually, leaving time for action for the server owner. While with
fully event based servers there are mostly only two modes of
performance: 100% performance and near-0% performance: total breakdown.

Ingo