DragonFly users List (threaded) for 2010-04
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Re: sysbench threading performance problems
:Pretty much defaullt:
:
:sysbench --num-threads=1 --test=fileio --file-test-mode=seqrd run
:
:Im not sure if the defaults are sane or insane. The only difference between
:these 2 boxes is the OS and the DF box has 2Gb but the FBSD has 4Gb RAM.
:
:Petr
You're hitting a cache blowout situation then. It works like this:
lets say the dataset fits in cache, and access time from the cache is
10uS. 100% of the data will be accessed in 10uS. Say 100 operations
are performed. The total time taken will be 100 * 10 = 1000uS.
Now lets say that the dataset is too large and 1% of it cannot be
cached in the best case. Access time for that 1% (if random I/O)
will be, say, 6mS (6000uS). Take 99 operations at 10uS and 1 operation
at 6000uS. The average per-operation access time will be
(99*10 + 1*6000) / 100 = 69uS.
In otherwords, blowing out the system caches by just 1% will reduce
performance by a factor of 6.9x in this example.
So any benchmark you do either has to be well within the bounds of
the cache, or well outside the bounds of the cache, in order for the
test to be reasonable. If the benchmark is near the cache boundary
you will get unusable results.
-Matt
Matthew Dillon
<dillon@backplane.com>
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