[datatable-help] Copy on assign broken in some cases

Muhammad Waliji mhwaliji at google.com
Sat Oct 29 02:42:45 CEST 2011


On Fri, Oct 28, 2011 at 5:32 PM, Matthew Dowle <mdowle at mdowle.plus.com>wrote:

>
> On Fri, 2011-10-28 at 09:52 -0700, Muhammad Waliji wrote:
> > >From the user's perspective, DT2 <- DT should either be a new copy or
> > a new reference.  Anything in between is confusing.
>
> Agreed. With picky caveat: even in base it's not at this point the copy
> is taken. It's later: copy-on-write. It's setkey and := that don't copy
> on write, not the (earlier) <-.
>

Hmm, I would prefer for these to have the same behavior.


>
> > How about this - add a new argument to data.table(), say max.cols.
> > max.cols defaults to a couple orders of magnitude above the initial
> > number of columns.  data.table allocates enough memory for max.cols
> > column pointers.  If you try to add more than max.cols columns, it is
> > either an error, or it creates a copy and produces a warning.
>
> Very nice idea. To over allocate by default so that := can add columns
> fully by reference most of the time seems good to me since there's a
> very low cost to over allocating the vector of column pointers. Create
> the (shallow copy) and issue a warning, I'm thinking, not error. The
> "max.cols" names seems a bit absolute, could it be "alloc.cols"?  We
> could have alloc(DT,2,ncol) or rowalloc(DT,n) and colalloc(DT,n), or
> realloc(...) so users can over alloc themselves before a loop that adds
> columns or inserts rows.  tables() could also report truenrow, and
> truencol as well as nrow and ncol.  What should alloc.cols be, by
> default? How about:  max(100,2*ncol)
>

Fine with me.

>
> What about as.data.table.data.frame()?  Should that over-allocate, too,
> or for speed just change the class attribute as it does now.
>

Yeah, I think any method of creating a data table should over-allocate.  If
people want the speed gains, they can set explicitly set alloc.cols.


> Maybe checking NAMED would work, in addition. If NAMED was 0, no need to
> warn. Only when NAMED was 1 (or 2) - (not too hot on NAMED) - would the
> warning be necessary.
>
>
> >
> > On Fri, Oct 28, 2011 at 1:10 AM, Matthew Dowle
> > <mdowle at mdowle.plus.com> wrote:
> >         Interesting one. Adding columns is a bit different to deleting
> >         and
> >         modifying columns. Here's how it works. Could make changes,
> >         could
> >         document it, or both, what do people think?
> >
> >         Just like data.frame there is a list vector holding pointers
> >         to the
> >         column vectors. A delete column op is done with a memmove to
> >         budge up
> >         the column pointers above the column by one place. That leaves
> >         a gap at
> >         the end. The length attribute of that vector (ncol(DT)) is
> >         then
> >         decremented and the spare 4 bytes (or 8 on 64bit) are left
> >         unused at the
> >         end.
> >
> >         An add column can't be fully by reference because the list
> >         vector is
> >         full. A new list vector has to be allocated, one slot larger,
> >         the old
> >         pointers memcpy'd over, and the last spot assigned the pointer
> >         to the
> >         new column vector.  This copying is negligible because it's a
> >         small list
> >         of pointers fitting well within one page. [Unless, there are
> >         many 1000's
> >         of columns, which is why it's done as efficiently as possible
> >         using
> >         memcpy].
> >
> >         Aside : There is little known (I guess) distinction between
> >         length and
> >         truelength in R internals. Base R doesn't use it, but we could
> >         in
> >         data.table. A delete column sets length but leaves truelength
> >         one
> >         larger. When the next add column comes along, it could just do
> >         the budge
> >         up and insert the column. That may not be so advantageous for
> >         (a small
> >         number) of columns,  but the same logic could work for
> >         insert() and
> >         delete()ing rows.  Of course, this would mean whether a
> >         visible copy or
> >         not is taken depends on what happened previously, rather than
> >         the
> >         syntax. That's something we've disliked before, in the same
> >         way we
> >         dislike drop=TRUE behaviour and so dropped drop. One way to
> >         approach
> >         this might be to advise ":= add *may* not copy. Best to assume
> >         it
> >         doesn't; use copy()". If you get in the habbit of
> >         "DT2=copy(DT)" then
> >         that'll take a deep copy at the time and you're safe.
> >
> >         To illustrate the partial (maybe shallow copy is better word),
> >         consider
> >         the following :
> >
> >         > DT = data.table(1:2,3:4)
> >         > DT2=DT
> >         > DT2[,y:=10L]
> >             V1 V2  y
> >         [1,]  1  3 10
> >         [2,]  2  4 10
> >         > DT
> >             V1 V2
> >         [1,]  1  3
> >         [2,]  2  4
> >         > DT2
> >             V1 V2  y
> >         [1,]  1  3 10
> >         [2,]  2  4 10
> >         > DT2[1,V1:=99L]
> >             V1 V2  y
> >         [1,] 99  3 10
> >         [2,]  2  4 10
> >         > DT
> >             V1 V2
> >         [1,] 99  3
> >         [2,]  2  4
> >         >
> >
> >         Matthew
> >
> >
> >         On Thu, 2011-10-27 at 11:46 -0700, Muhammad Waliji wrote:
> >         > I think this is a bug.  DT.2 <- DT.1 doesn't seem to make a
> >         copy in
> >         > all cases.
> >         >
> >         >
> >         > > DT.1 <- data.table(x=1, y=1)
> >         > > DT.2 <- DT.1
> >         > >
> >         > > # Both DT.1 and DT.2 are changed.
> >         > > DT.2[, y := NULL]
> >         >      x
> >         > [1,] 1
> >         > > DT.1
> >         >      x
> >         > [1,] 1
> >         > > DT.2
> >         >      x
> >         > [1,] 1
> >         > >
> >         > > # Only DT.2 is changed
> >         > > DT.2[, y := x]
> >         >      x y
> >         > [1,] 1 1
> >         > > DT.1
> >         >      x
> >         > [1,] 1
> >         > > DT.2
> >         >      x y
> >         > [1,] 1 1
> >         >
> >         >
> >
> >         > _______________________________________________
> >         > datatable-help mailing list
> >         > datatable-help at lists.r-forge.r-project.org
> >         >
> >
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
> >
> >
> >
> >
>
>
>
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