[Fwd: Re: Comparing the aio and epoll event frameworks.]

[jgmyers@netscape.com (John Myers)]

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Previously bounced due to some internal error on vger.


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Davide Libenzi wrote:

>
>Hi John, you seem to have lost a few episodes of the epoll saga. You can
>use epoll in both Edge Triggered or Level Triggered ways
>
I was aware of that.

>You can easily do thread pooling also.
>
Using epoll with thread pooling has the problems I describe.  You can 
get multiple threads simultaneously handling the same event.  This is 
particularly true when using epoll in level triggered mode.  
ep_reinject_items() reinjects items immediately before returning from 
sys_epoll_wait(), so any second thread calling epoll_wait() shortly 
thereafter is likely to also get a copy of the event.  In edge triggered 
mode, the window is significantly limited, but it is still there.

One can work around this issue by having user space maintain its own 
globally locked data structure containing its idea of the current epoll 
state, but this wastes CPU and becomes a likely site for locking 
contention.  The kernel is already serializing its own access to the 
struct eventpoll; user space should be able to exploit that.

>Is poll/select a single threading API ?
>
Yes.

> A thread pooling one ?
>
No.  You have to have a single thread calling poll/select on any given 
set of file descriptors.  The resulting events can then be farmed out to 
threads using some other synchronization method, but the API can only 
reasonably deliver events to that single calling thread.

Another difference I hadn't noticed before is that aio's 
sys_io_getevents() uses wake-one semantics, whereas epoll's 
sys_epoll_wait() appears to use wake-all semantics.  Wake-one semantics 
are important for thread pool callers in order to avoid thundering herd 
performance problems.  Aio unfortunately appears to wake up threads in 
FIFO order, which results in pessimal use of cache.  This should be 
changed to LIFO order.




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