Re: Out of memory error (even when using destroy())
On Saturday, 27 May 2017 at 17:57:03 UTC, Mike B Johnson wrote:
And what if one isn't interfacing to C? All pointers should be
known.
Apparently some people are (were?) working on semi-precise GC:
https://github.com/dlang/druntime/pull/1603
That still scans the stack conservatively, though.
Therefor, in a true D program(no outsourcing) with no pointers
used, the GC should never have to scan anything.
All realistic programs (in any language) use a lot of pointers -
for example, all slices in D have embedded pointers (slice.ptr),
references are pointers, classes are references, etc.
It seems the GC can be smarter than it is instead of just
making blanket assumptions about the entire program(which
rarely hold), which is generally always a poor choice when it
comes to performance...
If you only have compile time information, making blanket
assumptions is inevitable - after all, compiler can't understand
how a nontrivial program actually works. The alternative is doing
more work at runtime (marking pointers that changed since
previous collection, etc), which is also not good for performance.
Who knows, some pointer externally might be peeping in on our
hello world program.
Of course, there is a pointer :)
void main()
{
import std.stdio;
writeln("hello world".ptr);
}
Re: Out of memory error (even when using destroy())
On Saturday, 27 May 2017 at 17:57:03 UTC, Mike B Johnson wrote: And what if one isn't interfacing to C? All pointers should be known. You can't access memory by and int or any other non-pointer type! Hence, when pointers are created or ints are cast to pointers, the GC should be informed and then handle them appropriately Eh? So *every* cast from and to a pointer should become a call into the runtime, poking the GC? Or rather, every variable declaration should somehow be made magically known to the GC without any runtime cost? (then, instead of scanning a 100MB block of memory for "pointers" it should scan the list of possible pointers(which will generally be much much lower). That's precisely what it does, it scans the possible suspects, nothing more. That is, the stack (it has no idea what's there, it's just a block of untyped memory), memory it itself allocated *only if* it needs to (e.g. you allocated a typed array, and the type has pointers), memory you've specifically asked it to scan. It won't scan that block of 500k ints the OP allocated, unless told to do so. It would scan it if it was a void[] block though. Therefor, in a true D program(no outsourcing) with no pointers used, the GC should never have to scan anything. No pointers used? No arrays, no strings, no delegates?.. That's a rather limited program. But thing is, you're right, in such a program the GC will indeed never have to scan anything. If you never allocate, GC collection never occurs either. It seems the GC can be smarter than it is instead of just making blanket assumptions about the entire program(which rarely hold), which is generally always a poor choice when it comes to performance... Unnecessary interaction with the GC, e.g. informing it about every cast, is a poor choice for performance. After all, if we truly want to be safe, why not scan the entire memory of the system? Who knows, some pointer externally might be peeping in on our hello world program. What?
Re: Out of memory error (even when using destroy())
On Friday, 26 May 2017 at 18:19:48 UTC, H. S. Teoh wrote: On Fri, May 26, 2017 at 06:06:42PM +, Mike B Johnson via Digitalmars-d-learn wrote: On Friday, 26 May 2017 at 14:05:34 UTC, ag0aep6g wrote: > On 05/26/2017 10:15 AM, realhet wrote: > > But hey, the GC knows that is should not search for any > > pointers in those large blocks. And the buffer is full of > > 0-s at the start, so there can't be any 'false pointers' > > in it. And I think the GC will not search in it either. > > The issue is not that the block contains a false pointer, > but that there's a false pointer elsewhere that points into > the block. The bigger the block, the more likely it is that > something (e.g. an int on the stack) is mistaken for a > pointer into it. Wow, if that is the case then the GC has some real issues. The GC should be informed about all pointers and an int is not a pointer. Unfortunately, it can't, because (1) D interfaces with C code, and you don't have this kind of information from a C object file, and (2) you can turn a pointer into an int with a cast or a union in @system code, and since the GC cannot assume @safe for all code, it needs to be conservative and assume any int-like data could potentially be a pointer. You could improve GC performance by giving it type info from @safe code so that it skips over blocks that *definitely* have no pointers (it already does this to some extent, e.g., data in an int[] will never be scanned for pointers because the GC knows it can't contain any). But you can't make the GC fully non-conservative because it may crash the program when it wrongly assumes a memory block is dead when it's actually still live. All it takes is one pointer on the stack that's wrongly assumed to be just int, and you're screwed. And what if one isn't interfacing to C? All pointers should be known. You can't access memory by and int or any other non-pointer type! Hence, when pointers are created or ints are cast to pointers, the GC should be informed and then handle them appropriately(then, instead of scanning a 100MB block of memory for "pointers" it should scan the list of possible pointers(which will generally be much much lower). Therefor, in a true D program(no outsourcing) with no pointers used, the GC should never have to scan anything. It seems the GC can be smarter than it is instead of just making blanket assumptions about the entire program(which rarely hold), which is generally always a poor choice when it comes to performance... In fact, When interfacing with C or other programs, memory could be partitioned and any memory that may escape D is treated differently than the memory used only by D code. After all, if we truly want to be safe, why not scan the entire memory of the system? Who knows, some pointer externally might be peeping in on our hello world program.
Re: Out of memory error (even when using destroy())
On Fri, May 26, 2017 at 06:06:42PM +, Mike B Johnson via Digitalmars-d-learn wrote: > On Friday, 26 May 2017 at 14:05:34 UTC, ag0aep6g wrote: > > On 05/26/2017 10:15 AM, realhet wrote: > > > But hey, the GC knows that is should not search for any pointers > > > in those large blocks. And the buffer is full of 0-s at the > > > start, so there can't be any 'false pointers' in it. And I think > > > the GC will not search in it either. > > > > The issue is not that the block contains a false pointer, but that > > there's a false pointer elsewhere that points into the block. The > > bigger the block, the more likely it is that something (e.g. an int > > on the stack) is mistaken for a pointer into it. > > Wow, if that is the case then the GC has some real issues. The GC > should be informed about all pointers and an int is not a pointer. Unfortunately, it can't, because (1) D interfaces with C code, and you don't have this kind of information from a C object file, and (2) you can turn a pointer into an int with a cast or a union in @system code, and since the GC cannot assume @safe for all code, it needs to be conservative and assume any int-like data could potentially be a pointer. You could improve GC performance by giving it type info from @safe code so that it skips over blocks that *definitely* have no pointers (it already does this to some extent, e.g., data in an int[] will never be scanned for pointers because the GC knows it can't contain any). But you can't make the GC fully non-conservative because it may crash the program when it wrongly assumes a memory block is dead when it's actually still live. All it takes is one pointer on the stack that's wrongly assumed to be just int, and you're screwed. T -- Dogs have owners ... cats have staff. -- Krista Casada
Re: Out of memory error (even when using destroy())
On Friday, 26 May 2017 at 18:06:42 UTC, Mike B Johnson wrote: On Friday, 26 May 2017 at 14:05:34 UTC, ag0aep6g wrote: On 05/26/2017 10:15 AM, realhet wrote: But hey, the GC knows that is should not search for any pointers in those large blocks. And the buffer is full of 0-s at the start, so there can't be any 'false pointers' in it. And I think the GC will not search in it either. The issue is not that the block contains a false pointer, but that there's a false pointer elsewhere that points into the block. The bigger the block, the more likely it is that something (e.g. an int on the stack) is mistaken for a pointer into it. Wow, if that is the case then the GC has some real issues. The GC should be informed about all pointers and an int is not a pointer. What is a pointer if not an int? :) That is not an issue. The GC holds off releasing memory if there's even a suspicion that someone might be holding on to it. In most problems, ints are small. Pointers are always big, so there's not much overlap there. Accidents do happen occasionally, but it's better to have a system that is too cautious than one that ruins your data. Working with huge memory chunks isn't really a domain for GC though.
Re: Out of memory error (even when using destroy())
On Friday, 26 May 2017 at 14:05:34 UTC, ag0aep6g wrote: On 05/26/2017 10:15 AM, realhet wrote: But hey, the GC knows that is should not search for any pointers in those large blocks. And the buffer is full of 0-s at the start, so there can't be any 'false pointers' in it. And I think the GC will not search in it either. The issue is not that the block contains a false pointer, but that there's a false pointer elsewhere that points into the block. The bigger the block, the more likely it is that something (e.g. an int on the stack) is mistaken for a pointer into it. Wow, if that is the case then the GC has some real issues. The GC should be informed about all pointers and an int is not a pointer.
Re: Out of memory error (even when using destroy())
On 05/26/2017 10:15 AM, realhet wrote: But hey, the GC knows that is should not search for any pointers in those large blocks. And the buffer is full of 0-s at the start, so there can't be any 'false pointers' in it. And I think the GC will not search in it either. The issue is not that the block contains a false pointer, but that there's a false pointer elsewhere that points into the block. The bigger the block, the more likely it is that something (e.g. an int on the stack) is mistaken for a pointer into it.
Re: Out of memory error (even when using destroy())
Jordan Wilson wrote:
This I think achieves the spirit of your code, but without the
memory exception:
ubyte[] st;
foreach(i; 0..2000){
writeln(i);
st.length=500_000_000; // auto = new ubyte[500_000_000];
st.length=0; // destory(st)
st.assumeSafeAppend;
// prevent allocation by assuming it's ok to overrwrite what's
currently in st
}
Yea, that's the perfect solution. It uses exactly the amount of
memory that is required and still I'm using D things only.
The only difference is that I need only one variable outside of
the loop, but it's well worth it because I only need one large
buffer at a time.
Also refreshed my knowledge about assumeSafeAppend() which is now
clear to me, thanks to You.
Using this information I'll be able to do a BigArray class that
will hold large amount of data without worrying that the program
uses 3x more memory than needed :D
Thanks for everyone,
Such a helping community you have here!
Re: Out of memory error (even when using destroy())
On Friday, 26 May 2017 at 08:15:49 UTC, realhet wrote: 64bit is not a solution because I need to produce a 32bit dll, and I also wanna use 32bit asm objs. The total 2GB amount of memory is more than enough for the problem. My program have to produce 300..500 MB of continuous data frequently. This works in MSVC32, but with D's GC it starts to eat memory and fails at the 4th iteration. Actually it never releases the previous blocks even I say so with destroy(). If you have issues with false pointers, you can use malloc instead of the GC to use much less memory.
Re: Out of memory error (even when using destroy())
On Friday, 26 May 2017 at 06:31:49 UTC, realhet wrote:
Hi,
I'm kinda new to the D language and I love it already. :D So
far I haven't got any serious problems but this one seems like
beyond me.
import std.stdio;
void main(){
foreach(i; 0..2000){
writeln(i);
auto st = new ubyte[500_000_000];
destroy(st); //<-this doesnt matter
}
}
Compiled with DMD 2.074.0 Win32 it produces the following
output:
0
1
2
core.exception.OutOfMemoryError@src\core\exception.d(696):
Memory allocation failed
It doesn't matter that I call destroy() or not. This is ok
because as I learned: destroy only calls the destructor and
marks the memory block as unused.
But I also learned that GC will start to collect when it run
out of memory but in this time the following happens:
3x half GB of allocations and deallocations, and on the 4th the
system runs out of the 2GB
limit which is ok. At this point the GC already has 1.5GB of
free memory but instead of using that, it returns a Memory
Error. Why?
Note: This is not a problem when I use smaller blocks (like
50MB).
But I want to use large blocks, without making a slow wrapper
that emulates a large block by using smaller GC allocated
blocks.
Is there a solution to this?
Thank You!
I believe the general solution would be to limit allocation
within loops (given the issue Johnathan mentioned).
This I think achieves the spirit of your code, but without the
memory exception:
ubyte[] st;
foreach(i; 0..2000){
writeln(i);
st.length=500_000_000; // auto = new ubyte[500_000_000];
st.length=0; // destory(st)
st.assumeSafeAppend;
// prevent allocation by assuming it's ok to overrwrite what's
currently in st
}
Re: Out of memory error (even when using destroy())
On 26/05/2017 9:15 AM, realhet wrote:
Thanks for the answer!
But hey, the GC knows that is should not search for any pointers in
those large blocks.
And the buffer is full of 0-s at the start, so there can't be any 'false
pointers' in it. And I think the GC will not search in it either.
The only reference to the buffer is 'st' which will die shortly after it
has been allocated.
64bit is not a solution because I need to produce a 32bit dll, and I
also wanna use 32bit asm objs.
The total 2GB amount of memory is more than enough for the problem.
My program have to produce 300..500 MB of continuous data frequently.
This works in MSVC32, but with D's GC it starts to eat memory and fails
at the 4th iteration. Actually it never releases the previous blocks
even I say so with destroy().
At this point I only can think of:
a) Work with the D allocator but emulate large blocks by virtually
stitching small blocks together. (this is unnecessary complexity)
b) Allocating memory by Win32 api and not using D goodies anymore (also
unnecessary complexity)
But these are ugly workarounds. :S
I also tried to allocate smaller blocks than the previous one, so it
would easily fit to the prevouisly released space, and yet it keeps
eating memory:
void alloc_dealloc(size_t siz){
auto st = new ubyte[siz];
}
void main(){
foreach(i; 0..4) alloc_dealloc(500_000_000 - 50_000_000*i);
}
If you have to use such large amounts frequently, you really have to go
with buffers of memory that you control, not the GC. Memory allocation
is always expensive, if you can prevent it all the better.
Re: Out of memory error (even when using destroy())
Thanks for the answer!
But hey, the GC knows that is should not search for any pointers
in those large blocks.
And the buffer is full of 0-s at the start, so there can't be any
'false pointers' in it. And I think the GC will not search in it
either.
The only reference to the buffer is 'st' which will die shortly
after it has been allocated.
64bit is not a solution because I need to produce a 32bit dll,
and I also wanna use 32bit asm objs.
The total 2GB amount of memory is more than enough for the
problem.
My program have to produce 300..500 MB of continuous data
frequently. This works in MSVC32, but with D's GC it starts to
eat memory and fails at the 4th iteration. Actually it never
releases the previous blocks even I say so with destroy().
At this point I only can think of:
a) Work with the D allocator but emulate large blocks by
virtually stitching small blocks together. (this is unnecessary
complexity)
b) Allocating memory by Win32 api and not using D goodies anymore
(also unnecessary complexity)
But these are ugly workarounds. :S
I also tried to allocate smaller blocks than the previous one, so
it would easily fit to the prevouisly released space, and yet it
keeps eating memory:
void alloc_dealloc(size_t siz){
auto st = new ubyte[siz];
}
void main(){
foreach(i; 0..4) alloc_dealloc(500_000_000 - 50_000_000*i);
}
Re: Out of memory error (even when using destroy())
On Friday, May 26, 2017 06:31:49 realhet via Digitalmars-d-learn wrote:
> Hi,
>
> I'm kinda new to the D language and I love it already. :D So far
> I haven't got any serious problems but this one seems like beyond
> me.
>
> import std.stdio;
> void main(){
> foreach(i; 0..2000){
> writeln(i);
> auto st = new ubyte[500_000_000];
> destroy(st); //<-this doesnt matter
> }
> }
>
> Compiled with DMD 2.074.0 Win32 it produces the following output:
> 0
> 1
> 2
> core.exception.OutOfMemoryError@src\core\exception.d(696): Memory
> allocation failed
>
> It doesn't matter that I call destroy() or not. This is ok
> because as I learned: destroy only calls the destructor and marks
> the memory block as unused.
>
> But I also learned that GC will start to collect when it run out
> of memory but in this time the following happens:
> 3x half GB of allocations and deallocations, and on the 4th the
> system runs out of the 2GB
> limit which is ok. At this point the GC already has 1.5GB of
> free memory but instead of using that, it returns a Memory Error.
> Why?
>
> Note: This is not a problem when I use smaller blocks (like 50MB).
> But I want to use large blocks, without making a slow wrapper
> that emulates a large block by using smaller GC allocated blocks.
It's likely an issue with false pointers. The GC thinks that the memory is
referenced when it isn't, because some of the values match the pointers that
would need to be freed.
> Is there a solution to this?
Use 64-bit. False pointers don't tend to be a problem with 64-bit, whereas
they can be with 32-bit - especially when you're allocating large blocks of
memory like that.
- Jonathan M Davis
Out of memory error (even when using destroy())
Hi,
I'm kinda new to the D language and I love it already. :D So far
I haven't got any serious problems but this one seems like beyond
me.
import std.stdio;
void main(){
foreach(i; 0..2000){
writeln(i);
auto st = new ubyte[500_000_000];
destroy(st); //<-this doesnt matter
}
}
Compiled with DMD 2.074.0 Win32 it produces the following output:
0
1
2
core.exception.OutOfMemoryError@src\core\exception.d(696): Memory
allocation failed
It doesn't matter that I call destroy() or not. This is ok
because as I learned: destroy only calls the destructor and marks
the memory block as unused.
But I also learned that GC will start to collect when it run out
of memory but in this time the following happens:
3x half GB of allocations and deallocations, and on the 4th the
system runs out of the 2GB
limit which is ok. At this point the GC already has 1.5GB of
free memory but instead of using that, it returns a Memory Error.
Why?
Note: This is not a problem when I use smaller blocks (like 50MB).
But I want to use large blocks, without making a slow wrapper
that emulates a large block by using smaller GC allocated blocks.
Is there a solution to this?
Thank You!
