[BitTorrent] BT2 & hash trees

Olaf van der Spek OvdSpek at LIACS.NL
Fri Jul 23 08:38:01 EDT 2004

}T{Reme [Q_G] applsapff wrote:

>>>to calculate some hash? If it reduces the load on the servers, so be it.
>>>(within limits of course, no point having a user's computer
>>>so much it becomes unresponsive)
>>>To be honest, you hashed 4 gb of data. Part of that time is being used 
>>>retrieving the data from the hard disk. Unless you believe it 
>>Just reading data doesn't take that much CPU time.
> Yes I agree. Altho as far as I can tell those are total run times. Not total 

No, those are total CPU times, taken from Windows Task Manager.

> cpu time. Average drives can only read 30-50 megs a second. A little math 
> tells that about 41 to 68 percent of that time was added for calculating the 
> hash. As far as I can tell, a program is frozen during a read operation on a 

There's such a thing as read-ahead. The OS is smart enough to do a 
read-ahead, such that the second read operation is probably non-blocking.

> file (waiting for data, not doing anything) unless the test program was 
> using seperate threads to allow the calculation to continue while data was 
> being retrieved.  You can more or less assume that half of the time was 
> spent in a blocked state and the other processing. More acurate testing 
> could be done by hashing a pre-generated block of junk over and over.
> This does go a bit beyond the point tho...

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