Re: [PATCH] Decrease integer-share-limit
On Thu, 16 Aug 2012, Jakub Jelinek wrote: On Thu, Aug 16, 2012 at 04:44:03PM +0200, Richard Guenther wrote: On Thu, 16 Aug 2012, Paolo Carlini wrote: On 08/16/2012 03:39 PM, Richard Guenther wrote: This decreases the integer-share-limit to make sure the TREE_VEC we allocate for the small cached integers has a reasonable size for our GC memory allocator. Out of curiosity (just in case you hav two spare minutes) do you have any idea why this is so? I mean, naively one would think that allowing for any 8 bit constant would be a nice idea; puzzlingly, however the comment in the code says just experimentation. I'm wondering if tweaking a bit the memory allocator itself could allow for the full 8 bit range without a big memory waste... The GC memory allocator works on pages, there are not pages of arbitrary size but only power-of-two sizes. It's hard to improve the allocator here. The allocations are either power-of-two, or a couple of extra listed sizes. So, the alternative would be to add an extra size for the default integer share limit vector. See extra_order_size_table in ggc-page.c. Of course only provided there are many of those vectors. I doubt that a special size of 2080 / 1040 would be a good thing. (I doubt we make very good use of most of the small integer share vectors anyway, but that's for another thing ...) I've now applied the patch. Richard.
[PATCH] Decrease integer-share-limit
This decreases the integer-share-limit to make sure the TREE_VEC we
allocate for the small cached integers has a reasonable size for
our GC memory allocator. With the current value (256) and a reduced
testcase from PR54146 we see
tree.c:1224 (build_int_cst_wide) 40: 0.0%
0: 0.0% 35443680: 8.7% 11635920:39.0% 9305
thus 39% overhead (oops, that TREE_VEC has size 2080). Reducing
this to 251 yields
tree.c:1224 (build_int_cst_wide) 40: 0.0%
0: 0.0% 11698864: 3.0% 33696: 0.2% 9154
which is much nicer (size 2048 for signed ints, 1020 bytes on
32bit hosts if I counted correctly).
I'm queueing this for a bootstrap regtest run.
Richard.
2012-08-16 Richard Guenther rguent...@suse.de
* params.def (integer-share-limit): Decrease from 256 to 251,
add rationale.
Index: gcc/params.def
===
*** gcc/params.def (revision 190442)
--- gcc/params.def (working copy)
*** DEFPARAM(PARAM_MAX_LAST_VALUE_RTL,
*** 638,648
/* INTEGER_CST nodes are shared for values [{-1,0} .. N) for
{signed,unsigned} integral types. This determines N.
!Experimentation shows 256 to be a good value. */
DEFPARAM (PARAM_INTEGER_SHARE_LIMIT,
integer-share-limit,
The upper bound for sharing integer constants,
! 256, 2, 2)
DEFPARAM (PARAM_SSP_BUFFER_SIZE,
ssp-buffer-size,
--- 638,649
/* INTEGER_CST nodes are shared for values [{-1,0} .. N) for
{signed,unsigned} integral types. This determines N.
!Experimentation shows 251 to be a good value that generates the
!least amount of garbage for allocating the TREE_VEC storage. */
DEFPARAM (PARAM_INTEGER_SHARE_LIMIT,
integer-share-limit,
The upper bound for sharing integer constants,
! 251, 2, 2)
DEFPARAM (PARAM_SSP_BUFFER_SIZE,
ssp-buffer-size,
Re: [PATCH] Decrease integer-share-limit
Hi, On 08/16/2012 03:39 PM, Richard Guenther wrote: This decreases the integer-share-limit to make sure the TREE_VEC we allocate for the small cached integers has a reasonable size for our GC memory allocator. Out of curiosity (just in case you hav two spare minutes) do you have any idea why this is so? I mean, naively one would think that allowing for any 8 bit constant would be a nice idea; puzzlingly, however the comment in the code says just experimentation. I'm wondering if tweaking a bit the memory allocator itself could allow for the full 8 bit range without a big memory waste... Paolo.
Re: [PATCH] Decrease integer-share-limit
On Thu, 16 Aug 2012, Paolo Carlini wrote: Hi, On 08/16/2012 03:39 PM, Richard Guenther wrote: This decreases the integer-share-limit to make sure the TREE_VEC we allocate for the small cached integers has a reasonable size for our GC memory allocator. Out of curiosity (just in case you hav two spare minutes) do you have any idea why this is so? I mean, naively one would think that allowing for any 8 bit constant would be a nice idea; puzzlingly, however the comment in the code says just experimentation. I'm wondering if tweaking a bit the memory allocator itself could allow for the full 8 bit range without a big memory waste... The GC memory allocator works on pages, there are not pages of arbitrary size but only power-of-two sizes. It's hard to improve the allocator here. Richard.
Re: [PATCH] Decrease integer-share-limit
On Thu, Aug 16, 2012 at 04:44:03PM +0200, Richard Guenther wrote: On Thu, 16 Aug 2012, Paolo Carlini wrote: On 08/16/2012 03:39 PM, Richard Guenther wrote: This decreases the integer-share-limit to make sure the TREE_VEC we allocate for the small cached integers has a reasonable size for our GC memory allocator. Out of curiosity (just in case you hav two spare minutes) do you have any idea why this is so? I mean, naively one would think that allowing for any 8 bit constant would be a nice idea; puzzlingly, however the comment in the code says just experimentation. I'm wondering if tweaking a bit the memory allocator itself could allow for the full 8 bit range without a big memory waste... The GC memory allocator works on pages, there are not pages of arbitrary size but only power-of-two sizes. It's hard to improve the allocator here. The allocations are either power-of-two, or a couple of extra listed sizes. So, the alternative would be to add an extra size for the default integer share limit vector. See extra_order_size_table in ggc-page.c. Of course only provided there are many of those vectors. Jakub
