[web-devel] Caching the System time

Aristid Breitkreuz aristidb at googlemail.com
Mon Aug 8 14:41:59 CEST 2011

When I measured it, gettimeofday/clock_gettime took less than a microsecond.
Make sure the overhead of having a dedicated thread for time caching doesn't
slow things down. My test was on x86 Linux on some 2009 Intel Xeon.

Sent from my Android, please excuse the brevity.
Am 08.08.2011 14:25 schrieb "Gregory Collins" <greg at gregorycollins.net>:
> Not really. tryTakeMVar and tryPutMVar are pretty heavyweight and involve
> taking a mutex lock:
> http://hackage.haskell.org/trac/ghc/browser/rts/PrimOps.cmm#L1440
> IORef is much lighter-weight, see e.g.
> atomicModifyMutVar#:
> http://hackage.haskell.org/trac/ghc/browser/rts/PrimOps.cmm#L253
> and
> readMutVar# / writeMutVar#:
> The readMutVar primop just does a read (which turns into a small number of
> assembly instructions), and the writeMutVar primop just does a store and
> then marks the location dirty in the garbage collector. Doing an
> atomicModifyMutVar# involves a CAS which is also typically cheaper than
> mutex locking (you spin-loop instead of doing blocking/wakeup semantics).
> For the Snap date-caching code, we only mess with mutex locks if the date
> thread has been idle for a couple of seconds, in which case we don't mind
> paying the overhead of doing the locking. It's the "X hundred/thousand
> case we're trying to optimize.
> G
> On Sun, Aug 7, 2011 at 12:20 AM, Greg Weber <greg at gregweber.info> wrote:
>> couldn't tryTakeMVar and tryPutMVar handle this case of multiple writers
>> well? I am asking because I really don't know :)
>> On Sat, Aug 6, 2011 at 2:05 PM, Gregory Collins <greg at gregorycollins.net
>>> On Sat, Aug 6, 2011 at 8:11 PM, Michael Snoyman <michael at snoyman.com
>>>> Without having delved into the issue in any great depth, I had a
>>>> possible idea on this point. How about storing an IORef containing a
>>>> time and the text representations you need. Whenever you need to get
>>>> the time, compare the time in the IORef with the current time, and if
>>>> they're different, update appropriately. Bonus points: store in an
>>>> unpacked datatype and use a Builder instead of String. Would this (in
>>>> theory) work?
>>> We do something similar to this, except we get the thread to recompute
>>> date -- otherwise every second you have a race condition possibility
>>> multiple threads compete to write the new thread into the IORef. If
>>> no activity, the date thread goes to sleep, the code to get the current
>>> notices that the date is stale, and then it wakes the thread and
>>> the new date itself. Again, I'm not sure if it's worth it in the general
>>> case, but during periods of high load I think it helps to get as much
>>> out of the processing threads as possible.
>>> G
>>> --
>>> Gregory Collins <greg at gregorycollins.net>
>>> _______________________________________________
>>> web-devel mailing list
>>> web-devel at haskell.org
>>> http://www.haskell.org/mailman/listinfo/web-devel
> --
> Gregory Collins <greg at gregorycollins.net>
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