Le 03/05/2023 à 01:25, Henrik Bengtsson a écrit :
Along the lines of calling R_CheckUserInterrupt() only onces in a while:
OTOH, in the past we have had to *disable* R_CheckUserInterrupt()
in parts of R's code because it was too expensive,
{see current src/main/{seq.c,unique.c} for a series of commented-out
R_CheckUserInterrupt() for such speed-loss reasons}
First, here are links to these two files viewable online:
* https://github.com/wch/r-source/blob/trunk/src/main/seq.c
* https://github.com/wch/r-source/blob/trunk/src/main/unique.c
When not commented out, R_CheckUserInterrupt() would have been called
every 1,000,000 times per:
/* interval at which to check interrupts */
#define NINTERRUPT 1000000
and
if ((i+1) % NINTERRUPT == 0) R_CheckUserInterrupt()
in each iteration. That '(i+1) % NINTERRUPT == 0' expression can be
quite expensive too.
I vaguely remember a hack that was mentioned on this list as close to
0-cost. It looked something like:
iint = NINTERRUPT;
for (...) {
if (--iint == 0) {
R_CheckUserInterrupt();
iint = NINTERRUPT;
}
}
Best,
Serguei.
However, if we change the code to use NINTERRUPT
= 2^k where k = {1, 2, ...}, say
#define NINTERRUPT 1048576
the compiler would optimize the condition to use "the modulo of powers
of 2 can alternatively be expressed as a bitwise AND operation"
(Thomas Lumley, 2015-06-15). The speedup is quite impressive, cf.
<https://www.jottr.org/2015/06/05/checkuserinterrupt/>.
Alternatively, one can increment a counter and reset to zero after
calling R_CheckUserInterrupt(); I think that's equally performant.
Regarding making serialize() / unserialize() interruptible: I think
can be a good idea since we work with larger objects these days.
However, if we implement this, we probably have to consider what
happens when an interrupt happens. For example, transfers between a
client and a server are no longer atomic at this level, which means we
might end up in a corrupt state. This may, for instance, happen to
database transactions, and PSOCK parallel worker communication. A
quick fix would be to use base::suspendInterrupts(), but better would
of course be to handle interrupts gracefully.
My $.02 + $0.02
/Henrik
On Tue, May 2, 2023 at 3:56 PM Jeroen Ooms <jeroeno...@gmail.com> wrote:
On Tue, May 2, 2023 at 3:29 PM Martin Maechler
<maech...@stat.math.ethz.ch> wrote:
Ivan Krylov
on Tue, 2 May 2023 14:59:36 +0300 writes:
> В Sat, 29 Apr 2023 00:00:02 +0000
> Dario Strbenac via R-devel <r-devel@r-project.org> пишет:
>> Could save.image() be redesigned so that it promptly responds to
>> Ctrl+C? It prevents the command line from being used for a number of
>> hours if the contents of the workspace are large.
> This is ultimately caused by serialize() being non-interruptible. A
> relatively simple way to hang an R session for a long-ish time would
> therefore be:
> f <- xzfile(nullfile(), 'a+b')
> x <- rep(0, 1e9) # approx. 8 gigabytes, adjust for your RAM size
> serialize(x, f)
> close(f)
> This means that calling R_CheckUserInterrupt() between saving
> individual objects is not enough: R also needs to check for interrupts
> while saving sufficiently long vectors.
> Since the serialize() infrastructure is carefully written to avoid
> resource leaks on allocation failures, it looks relatively safe to
> liberally sprinkle R_CheckUserInterrupt() where it makes sense to do
> so, i.e. once per WriteItem() (which calls itself recursively and
> non-recursively) and once per every downstream for loop iteration.
> Valgrind doesn't show any new leaks if I apply the patch, interrupt
> serialize() and then exit. R also passes make check after the applied
> patch.
> Do these changes make sense, or am I overlooking some other problem?
Thank you, Ivan!
They do make sense... but :
OTOH, in the past we have had to *disable* R_CheckUserInterrupt()
in parts of R's code because it was too expensive,
{see current src/main/{seq.c,unique.c} for a series of commented-out
R_CheckUserInterrupt() for such speed-loss reasons}
so adding these may need a lot of care when we simultaneously
want to remain efficient for "morally valid" use of serialization...
where we really don't want to pay too much of a premium.
Alternatively, one could consider making R throttle or debounce calls
to R_CheckUserInterrupt such that a repeated calls within x time are
ignored, cf: https://www.freecodecamp.org/news/javascript-debounce-example/
The reasoning being that it may be difficult for (contributed) code to
determine when/where it is appropriate to check for interrupts, given
varying code paths and cpu speed. Maybe it makes more sense to call
R_CheckUserInterrupt frequently wherever it is safe to do so, and let
R decide if reasonable time has elapsed to actually run the (possibly
expensive) ui check again.
Basic example: https://github.com/r-devel/r-svn/pull/125/files
{{ saving the whole user workspace is not "valid" in that sense
in my view. I tell all my (non-beginner) Rstudio-using
students they should turn *off* the automatic saving and
loading at session end / beginning; and for reproducibility
only saveRDS() [or save()] *explicitly* a few precious
objects ..
}}
Again, we don't want to punish people who know what they are
doing, just because other R users manage to hang their R session
by too little thinking ...
Your patch adds 15 such interrupt checking calls which may
really be too much -- I'm not claiming they are: with our
recursive objects it's surely not very easy to determine the
"minimally necessary" such calls.
In addition, we may still consider adding an extra optional
argument, say `check.interrupt = TRUE`
which we may default to TRUE when save.image() is called
but e.g., not when serialize() is called..
Martin
> --
> Best regards,
> Ivan
> x[DELETED ATTACHMENT external: Rd_IvanKrylov_interrupt-serialize.patch,
text/x-patch]
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