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

[Dan Streetman]

On Mon, Nov 28, 2016 at 9:14 AM, Vitaly Wool <vitalywool@xxxxxxxxx> wrote:
> On Fri, Nov 25, 2016 at 7:25 PM, Dan Streetman <ddstreet@xxxxxxxx> wrote:
>> On Tue, Nov 15, 2016 at 11:00 AM, Vitaly Wool <vitalywool@xxxxxxxxx> 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.

which wait time?  for a page with the middle chunk mapped compact
should exit immediately...?

>
> ~vitaly
>
>>> Signed-off-by: Vitaly Wool <vitalywool@xxxxxxxxx>
>>> ---
>>>  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