code review 6094050: runtime: avoid unnecessary zeroization of huge memory blocks

runtime: avoid unnecessary zeroization of huge memory blocks
+move zeroization out of the heap mutex

[dvyukov, 2012-04-23 12:41:33]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://go.googlecode.com/hg/

[iant, 2012-05-02 00:25:53]

LGTM but wait for rsc

http://codereview.appspot.com/6094050/diff/8001/src/pkg/runtime/mcentral.c
File src/pkg/runtime/mcentral.c (right):

http://codereview.appspot.com/6094050/diff/8001/src/pkg/runtime/mcentral.c#ne...
src/pkg/runtime/mcentral.c:210: s = runtime·MHeap_Alloc(&runtime·mheap, npages,
c->sizeclass, 0, 1);
Just a note: since this code is going to walk the whole span anyhow to set up
the free list, it would be easy to mark each block as needing to be zeroed when
doing that.  I don't know if that would tend to be a win or not.

[rsc, 2012-05-02 01:26:38]

LGTM

[dvyukov, 2012-05-02 13:55:13]

On 2012/05/02 00:25:53, iant wrote:
> LGTM but wait for rsc
> 
> http://codereview.appspot.com/6094050/diff/8001/src/pkg/runtime/mcentral.c
> File src/pkg/runtime/mcentral.c (right):
> 
>
http://codereview.appspot.com/6094050/diff/8001/src/pkg/runtime/mcentral.c#ne...
> src/pkg/runtime/mcentral.c:210: s = runtime·MHeap_Alloc(&runtime·mheap,
npages,
> c->sizeclass, 0, 1);
> Just a note: since this code is going to walk the whole span anyhow to set up
> the free list, it would be easy to mark each block as needing to be zeroed
when
> doing that.  I don't know if that would tend to be a win or not.

Well, yes, perhaps mcentral can just mark all blocks as "needs to be zeroed".

I would also like to be able to ask mcache/mheap to not zeroize memory, but then
still get to know as to whether it's zeroized or not (i.e. you ask mheap to not
zeroize, but it says that the block is zeroized anyway).

I have another CL that reduces zeroization during sliceglow:
http://codereview.appspot.com/6100051/diff/5001/src/pkg/runtime/slice.c
It would benefit from such feature as well.

[dvyukov, 2012-05-02 14:01:33]

*** Submitted as http://code.google.com/p/go/source/detail?r=deddee9ec796 ***

runtime: avoid unnecessary zeroization of huge memory blocks
+move zeroization out of the heap mutex

R=golang-dev, iant, rsc
CC=golang-dev
http://codereview.appspot.com/6094050