Re: [PATCH] Elpa: Pinpoint semantics of `seq-subseq' for streams

["Clément Pit--Claudel" <clement.pit@gmail.com>]

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On 2016-09-14 20:51, John Mastro wrote:
> Clément Pit--Claudel <clement.pit@gmail.com> wrote:
>>> I think this is actually a very good example why it is good to
>>> forbid negative indexes.  If you are interested in the last n lines
>>> of a file, why would you dissect the complete file (or buffer) into
>>> lines and throw away nearly all of the result?
>>
>> Because it's much more memory-efficient, as long as the file's lines
>> are short :) Note that I was careful to say file, not buffer: I don't
>> need to load a full file in memory before I start processing its
>> lines. Same for the output of a running process: if I just want the
>> last n lines, then accumulating all of the output before going to the
>> end and looking backwards is extremely inefficient, memory-wise.
>> Dissecting the output (splitting it on newlines) and using a ring
>> buffer to keep only the last `n` ones is much better.
> 
> (Asking for my own edification)

:) Keep in mind that I could be making a mistake, too :)

> Wouldn't finding the last N elements require forcing every thunk in the
> stream (to find its end), thus using more memory than a linked list with
> the same contents?

That's correct, if by more memory you mean "more memory allocated over the lifetime of the process".  The key is that we don't need to allocate it all at once. In the simple case where we want, for example, just the last element, we only need to hold on to one value at a time.

> As long as you don't "hang on to the head" of the
> stream, earlier elements could be reclaimed by GC,

Exactly :)

> but the same applies to a list.

Not exactly: the list needs to be fully built before you iterate over it.  That's when the memory problems occur.
So yes, during iteration you can discard the cons cells that you've already seen in both cases, but in the list case these cells need to all be constructed and kept in memory beforehand.

> In short, I find this conversation interesting, but don't quite
> understand where the memory savings come in :)

Let me try to summarize it in a different way.  In the stream case, you build one cons cell at a time, and every time you build a new cons cell the previous one is available for garbage collection.  With a good GC, there's only a few cells physically present in memory at any time (plus the memory it takes to keep the last "n" elements, if you're desired output is the n-elements tail of the stream).

In the list case, on the other hand, the full list exists in memory before you iterate on it.  Sure, after you iterate on it, the list can be garbage collected; but before you iterate on it, all the cons cells need to exist at the same time.

Here's a concrete bit of code to demo this (I tried to write this in Emacs Lisp, but Emacs kept segfaulting on me, so I gave up and wrote it in Python):

    import sys

    def mkstream(n):
        for k in range(n):
            yield "a" * (k % 25) * 10

    def mklist(n):
        return ["a" * (k % 25) * 10 for k in range(n)]

    def last(seq):
        lastx = None
        for x in seq:
            lastx = x
        return lastx

    def test_list(n):
        last(mklist(n))

    def test_stream(n):
        last(mkstream(n))

    tests = {"stream": test_stream, "list": test_list}

    tests[sys.argv[1]](100*1000*1000)

When I run this on my machine as “python stream.py stream”, the total memory usage doesn't noticeably change.  When I run it as “python stream.py list”, python allocates about 25GB of RAM.  This obviously not exactly the same as what would happen on the Emacs Lisp side, but hopefully it's close enough :)

Clément.


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