CMUCL的奇怪的GC行为

Question

为什么这段代码溢出了CMUCL堆？即使我给它400MB内存(setq extensions:*bytes-consed-between-gcs* 400000000)CMUCL仍然会扼杀它.

; [GC threshold exceeded with 12,012,544 bytes in use.  Commencing GC.]
; [GC completed with 188,064 bytes retained and 11,824,480 bytes freed.]
; [GC will next occur when at least 400,188,064 bytes are in use.]
; [GC threshold exceeded with 400,202,280 bytes in use.  Commencing GC.]
; [GC completed with 207,120 bytes retained and 399,995,160 bytes freed.]
; [GC will next occur when at least 400,207,120 bytes are in use.]

这段代码可以运行CCL和SBCL,但我还没看过他们的内存使用情况.

这是CMUCL中的一个错误吗？我相信所有这些功能都是尾递归的.

(defun sqrt-iter (guess x)
  (if (good-enough? guess x)
      guess
      (sqrt-iter (improve guess x)
                 x)))

(defun improve (guess x)
  (average guess (/ x guess)))

(defun average (x y)
  (/ (+ x y) 2))

(defun good-enough? (guess x)
  (< (abs (- (* guess guess) x)) 0.001))

(defun mysqrt (x)
  (sqrt-iter 1.0 x))

(defun zint (x acc step)
  (setq num-iter (+ 1 num-iter))
  (if (>= x 10000.0)
      acc
      (zint (+ x step)
           (+ acc (* step (mysqrt x)))
           step)))

(setq num-iter 0)
(format t "result=~A; iterations=~A~%" (zint 0.0 0.0 .001) num-iter)
(quit)

编辑: 是的,CMUCL肯定很多,而且不必要.试试这个简单的例子:

$ ~/cmucl/bin/lisp 
...
* (defun foo () (bar))

FOO
* (defun bar () (foo))

BAR
* (foo)
; [GC threshold exceeded with 12,009,008 bytes in use.  Commencing GC.]
; [GC completed with 111,816 bytes retained and 11,897,192 bytes freed.]
; [GC will next occur when at least 12,111,816 bytes are in use.]
; [GC threshold exceeded with 12,120,912 bytes in use.  Commencing GC.]
; [GC completed with 120,016 bytes retained and 12,000,896 bytes freed.]
; [GC will next occur when at least 12,120,016 bytes are in use.]
; [GC threshold exceeded with 12,133,576 bytes in use.  Commencing GC.]

Answer 1

有两个函数是'tail recursive':sqr-iter和zint.

一般建议:

为了让编译器优化这些,DEBUG优化级别需要为2或更低.

您可以通过反汇编来查看编译器是否生成尾递归代码.使用该功能disassemble.

GC调用本身不是问题.大多数实现都不会在GC上打印任何内容.CMUCL默认打印(IIRC).CMUCL可能会为未优化的代码分配大量的浮点数 - 这可能会导致很多GC.

只有当CMUCL有堆栈溢出时,您才会看到尾调用优化不起作用.GC本身只显示大量的消费.

因此,要调试您的问题,首先需要查看是否使用尾调用优化编译代码.你可以反汇编代码.另一种选择是在运行调试器时放入代码,然后查看堆栈回溯.在堆栈上不应该有大量的递归调用 - 它们将被跳转替换.

如果代码以恒定的堆栈空间运行,那么您需要查看浮点数的分配.您需要检查代码不会分配太多的浮点数.