Re: [RFC] introduce --param max-lto-partition for having an upper bound on partition size

[Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>]

On 6 April 2016 at 14:54, Richard Biener <rguenther@suse.de> wrote:
> On Wed, 6 Apr 2016, Richard Biener wrote:
>
>> On Wed, 6 Apr 2016, Prathamesh Kulkarni wrote:
>>
>> > On 6 April 2016 at 13:44, Richard Biener <rguenther@suse.de> wrote:
>> > > On Wed, 6 Apr 2016, Prathamesh Kulkarni wrote:
>> > >
>> > >> On 5 April 2016 at 18:28, Richard Biener <rguenther@suse.de> wrote:
>> > >> > On Tue, 5 Apr 2016, Prathamesh Kulkarni wrote:
>> > >> >
>> > >> >> On 5 April 2016 at 16:58, Richard Biener <rguenther@suse.de> wrote:
>> > >> >> > On Tue, 5 Apr 2016, Prathamesh Kulkarni wrote:
>> > >> >> >
>> > >> >> >> On 4 April 2016 at 19:44, Jan Hubicka <hubicka@ucw.cz> wrote:
>> > >> >> >> >
>> > >> >> >> >> diff --git a/gcc/lto/lto-partition.c b/gcc/lto/lto-partition.c
>> > >> >> >> >> index 9eb63c2..bc0c612 100644
>> > >> >> >> >> --- a/gcc/lto/lto-partition.c
>> > >> >> >> >> +++ b/gcc/lto/lto-partition.c
>> > >> >> >> >> @@ -511,9 +511,20 @@ lto_balanced_map (int n_lto_partitions)
>> > >> >> >> >>    varpool_order.qsort (varpool_node_cmp);
>> > >> >> >> >>
>> > >> >> >> >>    /* Compute partition size and create the first partition.  */
>> > >> >> >> >> +  if (PARAM_VALUE (MIN_PARTITION_SIZE) > PARAM_VALUE (MAX_PARTITION_SIZE))
>> > >> >> >> >> +    fatal_error (input_location, "min partition size cannot be greater than max partition size");
>> > >> >> >> >> +
>> > >> >> >> >>    partition_size = total_size / n_lto_partitions;
>> > >> >> >> >>    if (partition_size < PARAM_VALUE (MIN_PARTITION_SIZE))
>> > >> >> >> >>      partition_size = PARAM_VALUE (MIN_PARTITION_SIZE);
>> > >> >> >> >> +  else if (partition_size > PARAM_VALUE (MAX_PARTITION_SIZE))
>> > >> >> >> >> +    {
>> > >> >> >> >> +      n_lto_partitions = total_size / PARAM_VALUE (MAX_PARTITION_SIZE);
>> > >> >> >> >> +      if (total_size % PARAM_VALUE (MAX_PARTITION_SIZE))
>> > >> >> >> >> +     n_lto_partitions++;
>> > >> >> >> >> +      partition_size = total_size / n_lto_partitions;
>> > >> >> >> >> +    }
>> > >> >> >> >
>> > >> >> >> > lto_balanced_map actually works in a way that looks for cheapest cutpoint in range
>> > >> >> >> > 3/4*parittion_size to 2*partition_size and picks the cheapest range.
>> > >> >> >> > Setting partition_size to this value will thus not cause partitioner to produce smaller
>> > >> >> >> > partitions only.  I suppose modify the conditional:
>> > >> >> >> >
>> > >> >> >> >       /* Partition is too large, unwind into step when best cost was reached and
>> > >> >> >> >          start new partition.  */
>> > >> >> >> >       if (partition->insns > 2 * partition_size)
>> > >> >> >> >
>> > >> >> >> > and/or in the code above set the partition_size to half of total_size/max_size.
>> > >> >> >> >
>> > >> >> >> > I know this is somewhat sloppy.  This was really just first cut implementation
>> > >> >> >> > many years ago. I expected to reimplement it marter soon, but then there was
>> > >> >> >> > never really a need for it (I am trying to avoid late IPA optimizations so the
>> > >> >> >> > partitioning decisions should mostly affect compile time performance only).
>> > >> >> >> > If ARM is more sensitive for partitining, perhaps it would make sense to try to
>> > >> >> >> > look for something smarter.
>> > >> >> >> >
>> > >> >> >> >> +
>> > >> >> >> >>    npartitions = 1;
>> > >> >> >> >>    partition = new_partition ("");
>> > >> >> >> >>    if (symtab->dump_file)
>> > >> >> >> >> diff --git a/gcc/lto/lto.c b/gcc/lto/lto.c
>> > >> >> >> >> index 9dd513f..294b8a4 100644
>> > >> >> >> >> --- a/gcc/lto/lto.c
>> > >> >> >> >> +++ b/gcc/lto/lto.c
>> > >> >> >> >> @@ -3112,6 +3112,12 @@ do_whole_program_analysis (void)
>> > >> >> >> >>    timevar_pop (TV_WHOPR_WPA);
>> > >> >> >> >>
>> > >> >> >> >>    timevar_push (TV_WHOPR_PARTITIONING);
>> > >> >> >> >> +
>> > >> >> >> >> +  if (flag_lto_partition != LTO_PARTITION_BALANCED
>> > >> >> >> >> +      && PARAM_VALUE (MAX_PARTITION_SIZE) != INT_MAX)
>> > >> >> >> >> +    fatal_error (input_location, "--param max-lto-partition should only"
>> > >> >> >> >> +              " be used with balanced partitioning\n");
>> > >> >> >> >> +
>> > >> >> >> >
>> > >> >> >> > I think we should wire in resonable MAX_PARTITION_SIZE default.  THe value you
>> > >> >> >> > found experimentally may be a good start. For that reason we can't really
>> > >> >> >> > refuse a value when !LTO_PARTITION_BALANCED.  Just document it as parameter for
>> > >> >> >> > balanced partitioning only and add a parameter to lto_balanced_map specifying whether
>> > >> >> >> > this param should be honored (because the same path is used for partitioning to one partition)
>> > >> >> >> >
>> > >> >> >> > Otherwise the patch looks good to me modulo missing documentation.
>> > >> >> >> Thanks for the review. I have updated the patch.
>> > >> >> >> Does this version look OK ?
>> > >> >> >> I had randomly chosen 10000, not sure if that's an appropriate value
>> > >> >> >> for default.
>> > >> >> >
>> > >> >> > I think it's way too small.  This is roughly the number of GIMPLE stmts
>> > >> >> > (thus roughly the number of instructions).  So with say a 8 byte
>> > >> >> > instruction format it is on the order of 80kB.  You'd want to have a
>> > >> >> > default of at least several ten times of large-unit-insns (also 10000).
>> > >> >> > I'd choose sth like 1000000 (one million).  I find the lto-min-partition
>> > >> >> > number quite small as well (and up it by a factor of 10).
>> > >> >> Done in this version.
>> > >> >
>> > >> > I'd do that separately.
>> > >> >
>> > >> > Please no default parameter for lto_balanced_map (), instead change
>> > >> > all callers.
>> > >> >
>> > >> >> Is it OK after bootstrap+test ?
>> > >> >
>> > >> > Note that this is for stage1 only.  I'll leave approval to Honza
>> > >> > (also verification of the default max param - not sure if for example
>> > >> > chromium or firefox should/will be split to more than 32 partitions
>> > >> > with the patch)
>> > >> Removed default parameter in this version. I verified with the patch
>> > >> for chromium LTO build:
>> > >> n_lto_partitions == 32, ltrans_partitions.length() == 31
>> > >
>> > > Just noticed that lto_balanced_map already gets PARAM_LTO_PARTITIONS,
>> > > so why not pass it PARAM_MAX_PARTITION_SIZE or 0 (as magic value for
>> > > unlimited) instead of a bool parameter?
>> > Indeed.  Instead of 0, would it be OK to pass INT_MAX as 2nd parameter in case
>> > of single partition, since in that case partition->insns >
>> > max_partition_size will never
>> > be true, which would effectively ignore max_partition_size.
>>
>> You mean we are limited to INT_MAX partition size anyway, even on 64bit
>> systems? ...  (but yes, using a suitable large number works as well)
>
> Ah, even 'total_size' is an int ... I wonder what this means for LTOing
> a -mcmodel=large app (that really needs the large model).
Hi,
Is the attached patch OK for trunk now ?
Bootstrapped and tested on x86_64-unknown-linux-gnu.
Cross tested on arm*-*-* and aarch64*-*-*.

Thanks,
Prathamesh
>
> Richard.

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