linux-kernel - Re: [PATCH 3/3] z3fold: discourage use of pages that weren't compacted

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Date:   Mon, 28 Nov 2016 15:14:40 +0100
From:   Vitaly Wool <vitalywool@...il.com>
To:     Dan Streetman <ddstreet@...e.org>
Cc:     Linux-MM <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] z3fold: discourage use of pages that weren't compacted

On Fri, Nov 25, 2016 at 7:25 PM, Dan Streetman <ddstreet@...e.org> wrote:
> On Tue, Nov 15, 2016 at 11:00 AM, Vitaly Wool <vitalywool@...il.com> wrote:
>> If a z3fold page couldn't be compacted, we don't want it to be
>> used for next object allocation in the first place.
>
> why?  !compacted can only mean 1) already compact or 2) middle chunks
> is mapped.  #1 is as good compaction-wise as the page can get, so do
> you mean that if a page couldn't be compacted because of #2, we
> shouldn't use it for next allocation?  if so, that isn't quite what
> this patch does.
>
>> It makes more
>> sense to add it to the end of the relevant unbuddied list. If that
>> page gets compacted later, it will be added to the beginning of
>> the list then.
>>
>> This simple idea gives 5-7% improvement in randrw fio tests and
>> about 10% improvement in fio sequential read/write.
>
> i don't understand why there is any improvement - the unbuddied lists
> are grouped by the amount of free chunks, so all pages in a specific
> unbuddied list should have exactly that number of free chunks
> available, and it shouldn't matter if a page gets put into the front
> or back...where is the performance improvement coming from?

When the next attempt to compact this page comes, it's less likely
it's locked so the wait times are slightly lower in average.

~vitaly

>> Signed-off-by: Vitaly Wool <vitalywool@...il.com>
>> ---
>>  mm/z3fold.c | 22 +++++++++++++++++-----
>>  1 file changed, 17 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/z3fold.c b/mm/z3fold.c
>> index ffd9353..e282ba0 100644
>> --- a/mm/z3fold.c
>> +++ b/mm/z3fold.c
>> @@ -539,11 +539,19 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
>>                 free_z3fold_page(zhdr);
>>                 atomic64_dec(&pool->pages_nr);
>>         } else {
>> -               z3fold_compact_page(zhdr);
>> +               int compacted = z3fold_compact_page(zhdr);
>>                 /* Add to the unbuddied list */
>>                 spin_lock(&pool->lock);
>>                 freechunks = num_free_chunks(zhdr);
>> -               list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
>> +               /*
>> +                * If the page has been compacted, we want to use it
>> +                * in the first place.
>> +                */
>> +               if (compacted)
>> +                       list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
>> +               else
>> +                       list_add_tail(&zhdr->buddy,
>> +                                     &pool->unbuddied[freechunks]);
>>                 spin_unlock(&pool->lock);
>>                 z3fold_page_unlock(zhdr);
>>         }
>> @@ -672,12 +680,16 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
>>                                 spin_lock(&pool->lock);
>>                                 list_add(&zhdr->buddy, &pool->buddied);
>>                         } else {
>> -                               z3fold_compact_page(zhdr);
>> +                               int compacted = z3fold_compact_page(zhdr);
>>                                 /* add to unbuddied list */
>>                                 spin_lock(&pool->lock);
>>                                 freechunks = num_free_chunks(zhdr);
>> -                               list_add(&zhdr->buddy,
>> -                                        &pool->unbuddied[freechunks]);
>> +                               if (compacted)
>> +                                       list_add(&zhdr->buddy,
>> +                                               &pool->unbuddied[freechunks]);
>> +                               else
>> +                                       list_add_tail(&zhdr->buddy,
>> +                                               &pool->unbuddied[freechunks]);
>>                         }
>>                 }
>>
>> --
>> 2.4.2