On Tue, 2014-08-05 at 15:32 -0700, [email protected] wrote: > Jameson, > > > I disagree that my numbers contradict my claim. Here are my numbers > presented more succinctly: > > > type immutable > no update 0.14 0.04 > fast update 0.62 not available > slow update 4.51 0.35 > > > The difference I am complaining about is between 'fast update' and > 'slow update' for 'immutable'. Since fast update is not available for > 'immutable', we can only guess how much improvement is possible. > However, judging from the improvement between slow and fast update > for 'type', it seems likely to me that there would also be a > noticeable difference for 'immutable.' > > > As for llvm, I don't know what its properties and limitations are. > However, it seems to me that a more intuitive way to deal with the > issue I raise (compared to current Julia) is to deprecate the current > usage of 'immutable' and instead split the two attributes into two > keywords: > > > type A : refcounted # like current 'type' > end > type A : immutable # like current 'immutable' > end > type A : refcounted, immutable # not currently available > end > type A # i.e., neither refcounted nor immutable; not currently > available. > end > Umh, did you miss the part about julia not using reference counting? Reference counting is an extremely common method to use when managing memory by hand and an extremely uncommon technique to use by systems that provide automatic garbage collection.
Ross Boylan > > > > > On Tuesday, August 5, 2014 5:38:17 PM UTC-4, [email protected] > wrote: > Dear Julia users, > > > It seems to me that Julia's distinction between a 'type' and > an 'immutable' conflates two independent properties; the > consequence of this conflation is a needless loss of > performance. In more detail, the differences between a 'type' > struct and 'immutable' struct in Julia are: > > > 1. Assignment of 'type' struct copies a pointer; assignment of > an 'immutable' struct copies the data. > > > 2. An array of type structs is an array of pointers, while an > array of immutables is an array of data. > > > 3. Type structs are refcounted, whereas immutables are not. > (This is not documented; it is my conjecture.) > > > 4. Fields in type structs can be modified, but fields in > immutables cannot. > > > Clearly #1-#3 are related concepts. As far as I can see, #4 > is completely independent from #1-#3, and there is no obvious > reason why it is forbidden to modify fields in immutables. > There is no analogous restriction in C/C++. > > > This conflation causes a performance hit. Consider: > > > type floatbool > a::Float64 > b:Bool > end > > > If t is of type Array{floatbool,1} and I want to update the > flag b in t[10] to 'true', I say 't[10].b=true' (call this > 'fast'update). But if instead of 'type floatbool' I had said > 'immutable floatbool', then to set flag b in t[10] I need the > more complex code t[10] = floatbool(t[10].a,true) (call this > 'slow' update). > > > To document the performance hit, I wrote five functions below. > The first three use 'type' and either no update, fast update, > or slow update; the last two use 'immutable' and either no > update or slow update. You can see a HUGE hit on performance > between slow and fast update for `type'; for immutable there > would presumably also be a difference, although apparently > smaller. (Obviously, I can't test fast update for immutable; > this is the point of my message!) > > > So why does Julia impose this apparently needless restriction > on immutable? > > > -- Steve Vavasis > > > > > julia> @time testimmut.type_upd_none() > @time testimmut.type_upd_none() > elapsed time: 0.141462422 seconds (48445152 bytes allocated) > > > julia> @time testimmut.type_upd_fast() > @time testimmut.type_upd_fast() > elapsed time: 0.618769232 seconds (48247072 bytes allocated) > > > julia> @time testimmut.type_upd_slow() > @time testimmut.type_upd_slow() > elapsed time: 4.511306586 seconds (4048268640 bytes allocated) > > > julia> @time testimmut.immut_upd_none() > @time testimmut.immut_upd_none() > elapsed time: 0.04480173 seconds (32229468 bytes allocated) > > > julia> @time testimmut.immut_upd_slow() > @time testimmut.immut_upd_slow() > elapsed time: 0.351634871 seconds (32000096 bytes allocated) > > > module testimmut > > > type xytype > x::Int > y::Float64 > z::Float64 > summed::Bool > end > > > immutable xyimmut > x::Int > y::Float64 > z::Float64 > summed::Bool > end > > > myfun(x) = x * (x + 1) * (x + 2) > > > function type_upd_none() > n = 1000000 > a = Array(xytype, n) > for i = 1 : n > a[i] = xytype(div(i,2), 0.0, 0.0, false) > end > numtri = 100 > for tri = 1 : numtri > sum = 0 > for i = 1 : n > @inbounds x = a[i].x > sum += myfun(x) > end > end > end > > > > > function type_upd_fast() > n = 1000000 > a = Array(xytype, n) > for i = 1 : n > a[i] = xytype(div(i,2), 0.0, 0.0, false) > end > numtri = 100 > for tri = 1 : numtri > sum = 0 > for i = 1 : n > @inbounds x = a[i].x > sum += myfun(x) > @inbounds a[i].summed = true > end > end > end > > > function type_upd_slow() > n = 1000000 > a = Array(xytype, n) > for i = 1 : n > a[i] = xytype(div(i,2), 0.0, 0.0, false) > end > numtri = 100 > for tri = 1 : numtri > sum = 0 > for i = 1 : n > @inbounds x = a[i].x > sum += myfun(x) > @inbounds a[i] = xytype(a[i].x, a[i].y, a[i].z, > true) > end > end > end > > > > > function immut_upd_none() > n = 1000000 > a = Array(xyimmut, n) > for i = 1 : n > a[i] = xyimmut(div(i,2), 0.0, 0.0, false) > end > numtri = 100 > for tri = 1 : numtri > sum = 0 > for i = 1 : n > @inbounds x = a[i].x > sum += myfun(x) > end > end > end > > > function immut_upd_slow() > n = 1000000 > a = Array(xyimmut, n) > for i = 1 : n > a[i] = xyimmut(div(i,2), 0.0, 0.0, false) > end > numtri = 100 > for tri = 1 : numtri > sum = 0 > for i = 1 : n > @inbounds x = a[i].x > sum += myfun(x) > @inbounds a[i] = xyimmut(a[i].x, a[i].y, a[i].z, > true) > end > end > end > > > end > > > > >
