Lew wrote:
> Tom Anderson wrote:
>> If someone (credible) told me that there was a five-to-one difference
>> in times for an ArrayList.get and an array[], then, if we were in a
>> context where performance was im****tant, i certainly would take that
>> as kind of damning for collections. I think that's perfectly
reasonable.
>
> Eric is correct that you should not generalize from a single data point,
> especially when full details are not available for that one case. I
> presented the anecdote not to prove anything but to provide evidence, at
> least that the matter is worth pursuing more objectively.
>
> My comment that I lack credibility was not about personality but about
> evidence and what makes it valid. I provided no hard data, nor enough
> information about the anecdote to make it something on which you can
> base conclusions. By hard standards, I indeed do lack credibility to
> assert that direct-to-disk is five times faster. OTOH, it is enough to
> share one experience that indicates that the claim is potentially
> sup****table, just maybe.
It depends on how much effort you're willing to put in. A major reason
for databases running faster when off the file system, for example, is
doing things like putting SQL tables into a continuous range of
cylinders, and putting the lowest level of the primary index on track
zero of those same cylinders. Then, to find a given record, you go
through the high-level parts of the index tree, which are probably
cached, anyway; move the access arm to the correct cylinder; read the
low-level index record from track zero (perhaps it is replicated as many
times as it will fit on the track, so that it comes up as fast as
possible); and then read the data without moving the access arm again.
That gives you indexed access only imperceptibly slower than hashed.
Much-larger-than-RAM sorts can use similar tricks, such as merging
RAM-sorted strings from the top half of the disk to the bottom half and
then back again, until the final merge to the target.
IBM mainframes have used methods like this since the mid 1960s.
If, on the other hand, you're just doing the equivalent of reading a
FileInputStream or writing a FileOutputStream, stick with the file
system. Moving it off will accomplish almost nothing.
--
John W. Kennedy
"Give up vows and dogmas, and fixed things, and you may grow like
That. ...you may come to think a blow bad, because it hurts, and not
because it humiliates. You may come to think murder wrong, because it
is violent, and not because it is unjust."
-- G. K. Chesterton. "The Ball and the Cross"
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