On Saturday, October 22, 2016 at 11:59:55 PM UTC+8, Ian Lance Taylor wrote:
>
> On Sat, Oct 22, 2016 at 2:01 AM, T L <tapi...@gmail.com <javascript:>>
> wrote:
> >
> > On Saturday, October 22, 2016 at 4:57:52 PM UTC+8, T L wrote:
> >>
> >>
> >> The string struct used internally is
> >>
> >> type stringStruct struct {
> >> str unsafe.Pointer
> >> len int
> >> }
> >>
> >> When following f function is called and s is cleared,
> >> how do go runtime knows the starting memory address of the old s.str is
> >> "a" instead of "c"?
> >
> >
> > I mean how do go runtime knows "abcdefg" instead of "cde" should be
> > released.
>
> The garbage collector works in terms of allocated memory blocks. If
> there is a pointer into the memory block, it is retained. If there
> are no remaining pointers to the block, it is released. The fact that
> the string points into the middle of the block only matters in that it
> causes the block to be retained. Where the string points has no
> effect on what is released when there are no remaining pointers to the
> block.
>
So a string always occupy most one block, no matter how large the string is?
>
> Note that this does mean that in the current GC implementation a
> pointer into the middle of a string will cause the entire string to be
> retained, even though if there are no pointers to other parts of the
> string then there is no way to use that memory. In other words,
> taking a small slice of a large string will cause the large string to
> remain in memory as long as the small slice is live. I'm not aware of
> any current plans to fix this (and I don't even know how it could be
> fixed without significantly decreasing throughput). It's hard to know
> how big a problem this is, as people are aware of it and write their
> programs to avoid it. This is https://golang.org/issue/9618.
>
> Ian
>
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