[OpenAFS-devel] .35 sec rx delay bug?

[Roland Kuhn]

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Hi Ken!

On 9 Nov 2006, at 21:21, Ken Hornstein wrote:

>>> Hm, I am wondering if that's a win; that means two extra syscalls.
>>
>> Here two extra syscalls (a few microseconds nowadays?) can save  
>> processing
>> a packet (or more), and incurring network latency (a hundred  
>> microseconds
>> on my Ether).
>
> The syscalls (context switch, a good chunk of cache getting flushed)
> add up.  Why do you think pread() and pwrite() were created?  My point
> was that the extra syscalls may end up killing the advantage you get
> from sendfile() (the Linux sendfile() ... other sendfiles can add
> in header data).  But ... that's just thinking out loud.  Maybe it
> will be fine.
>
I'm not a guru, but I think that's not correct. pread/pwrite are  
there to prevent some races, not to save time. And at least in Linux  
syscalls are _fast_, because there is no context switch (that term is  
reserved for the switch between processes, as that is a _lot_ slower  
than entering/exiting kernel space). That's the whole point of  
mapping kernel address space into each vm (with proper protection, of  
course).

A syscall is of the order of 1usec, which is much shorter than the  
network stack latencies which are being talked about in this thread.

Ciao,
                     Roland

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