Comment by no-bugs

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Comment by no-bugs

> Honestly, it's a lot more complicated than that, and the article itself presents a pretty complicated (and flawed) model. There's no substituting for benchmarks.

Of course, all the models are inherently flawed, and in theory there is no substituting for benchmarks, but going along the lines of "all models out there are flawed, so we don't need ANY model and should do benchmarking instead" leads to the software which is badly architectured to start with (and then no benchmarking will be able heal it without throwing the whole thing away and rewriting it from scratch - and it is not going to happen for most of the projects for many practical reasons).

In other words, if we're already in the vicinity of the optimum (which BTW may happen to be a local one - but this is a very different story) - then benchmarking is the way to go (that is, if those 20% we're chasing, are worth the trouble - and TBH, they really are). However, at the earlier stages being able to realise the order of magnitude without spending time on trying really badly architectured things - is extremely important.

Just one example - no optimisation will save a (non-GPGPU) program which tries to split the load over multiple threads with chunks calculated on each thread (before it relinquishes control to another thread), being 100 cycles or so; I don't need to benchmark it to know that at least in 99.9999% of real-world projects it is a suicide from performance point of view - and this observation directly follows from the diagram.

Or another example - in most of practical cases (actually, pretty much always except for inserts into the middle without iterating through and specialized stuff such as merges), vectors have advantage over lists - and this advantage can easily reach over 100x(!) ( see, for example, https://accu.org/index.php/journals/2268 ). This, once again, follows from the diagram (it is a direct consequence of the main memory read being over 100 cycles and L1 read being 3-4 cycles).

This list of crazily inefficient things (which are still widely used in the industry for no reason) can go on and on. Of course, going into anywhere more detailed analysis than "order of magnitude" indeed doesn't make sense - and it is clearly stated in OP. But for initial estimates when making some architectural decisions (and to avoid premature pessimisation) - such "order of magnitude" analysis can be very useful.

Replies no-bugs · 2016-11-12
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