MySQL Performance 6.0rc1

[Robert Watson]

On Thu, 27 Oct 2005, Poul-Henning Kamp wrote:

>>> Why would anybody take a timestamp at all I/O syscalls ?
>>>
>>> "I wonder why my car can only go 30 km/h with the trunk full of 
>>> concrete" ?
>>>
>>> In a data base application I could possibly understand a timestamp 
>>> after every write.
>>>
>>> But after _all_ I/O syscalls ?  That's just plain stupid...
>>
>> Don't panic, I agree that is stupid code, but I can not change it, it 
>> is not written by me, sorry!
>
> I'm not panicing, I'm merely pointing out that we should not optimize 
> performance after bogus code but rather try to improve it.

There is, of course, a tension in optimizing systems to speed up 
applications that have been optimized for other systems. :-)

Sadly, POSIX doesn't say anything about how applications can express 
preferences about the cost and granularity of time measurement.  It's long 
been an issue though: AFS used to do magic on many UNIX systems to expose 
a timer tick timestamp to user space from the kernel via a special magic 
page.  This trick has been used lots of other places to, but worked on the 
assumption that an application would be willing to pay a lower cost to get 
a poor measurement of time frequently, rather than pay a higher cost to 
get a time measurement less frequently.  For user space applications 
implementing network protocols, this is important since they do need 
accurate measurements of round-trip time in order to calculate bandwidth 
windows, etc.  We almost want clock_gettime() with CLOCK_TENMS, or the 
like.

Robert N M Watson