Clojure

Applying a var to an infinite arglist consumes all available memory

Details

  • Type: Defect Defect
  • Status: Open Open
  • Priority: Major Major
  • Resolution: Unresolved
  • Affects Version/s: None
  • Fix Version/s: None
  • Component/s: None
  • Labels:
  • Patch:
    Code and Test
  • Approval:
    Prescreened

Description

It is possible to apply a function to an infinite argument list: for example, (apply distinct? (repeat 1)) immediately returns false, after realizing just a few elements of the infinite sequence (repeat 1). However, (apply #'distinct? (repeat 1)) attempts to realize all of (repeat 1) into memory at once.

This happens because Var.applyTo delegates to AFn.applyToHelper to decide which arity of Var.invoke to dispatch to; but AFn doesn't expect infinite arglists (mostly those use RestFn). So it uses RT.seqToArray, which doesn't work well in this case.

Instead, Var.applyTo(args) can just dispatch to deref().applyTo(args), and let the function being stored figure out what to do with the arglist.

I've changed Var.applyTo to do this, and added a test (which fails before my patch is applied, and passes afterwards).

Patch: clj-1423.patch

Screened by: Alex Miller

  1. apply-var.patch
    15/May/14 4:59 PM
    2 kB
    Alan Malloy
  2. clj-1423.patch
    18/Jan/16 5:29 PM
    2 kB
    Alex Miller

Activity

Alex Miller made changes -
Field Original Value New Value
Approval Triaged [ 10120 ]
Alex Miller made changes -
Priority Minor [ 4 ] Major [ 3 ]
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Alex Miller added a comment - - edited

I also did a quick perf test on this:

(dotimes [x 20] (time (dotimes [y 10000] (apply #'+ (range 100)))))

which showed times ~82 ms per rep before the patch and ~10 ms per rep after the patch (on par with applying the function directly).

I added a new patch that squashes the commits, adds the ticket number to the commit message, attribution was retained.

Show
Alex Miller added a comment - - edited I also did a quick perf test on this:
(dotimes [x 20] (time (dotimes [y 10000] (apply #'+ (range 100)))))
which showed times ~82 ms per rep before the patch and ~10 ms per rep after the patch (on par with applying the function directly). I added a new patch that squashes the commits, adds the ticket number to the commit message, attribution was retained.
Alex Miller made changes -
Approval Triaged [ 10120 ] Prescreened [ 10220 ]
Description It is possible to apply a function to an infinite argument list: for example, (apply distinct? (repeat 1)) immediately returns false, after realizing just a few elements of the infinite sequence (repeat 1). However, (apply #'distinct? (repeat 1)) attempts to realize all of (repeat 1) into memory at once.

This happens because Var.applyTo delegates to AFn.applyToHelper to decide which arity of Var.invoke to dispatch to; but AFn doesn't expect infinite arglists (mostly those use RestFn). So it uses RT.seqToArray, which doesn't work well in this case.

Instead, Var.applyTo(args) can just dispatch to deref().applyTo(args), and let the function being stored figure out what to do with the arglist.

I've changed Var.applyTo to do this, and added a test (which fails before my patch is applied, and passes afterwards).
It is possible to apply a function to an infinite argument list: for example, (apply distinct? (repeat 1)) immediately returns false, after realizing just a few elements of the infinite sequence (repeat 1). However, (apply #'distinct? (repeat 1)) attempts to realize all of (repeat 1) into memory at once.

This happens because Var.applyTo delegates to AFn.applyToHelper to decide which arity of Var.invoke to dispatch to; but AFn doesn't expect infinite arglists (mostly those use RestFn). So it uses RT.seqToArray, which doesn't work well in this case.

Instead, Var.applyTo(args) can just dispatch to deref().applyTo(args), and let the function being stored figure out what to do with the arglist.

I've changed Var.applyTo to do this, and added a test (which fails before my patch is applied, and passes afterwards).

*Patch:* clj-1423.patch

*Screened by:* Alex Miller
Attachment clj-1423.patch [ 15395 ]
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  • Created:
    Updated: