[PATCH] mm/zsmalloc: avoid unnecessary iteration in get_pages_per_zspage()

[PATCH] mm/zsmalloc: avoid unnecessary iteration in get_pages_per_zspage()

[Ganesh Mahendran]

if we find a zspage with usage == 100%, there is no need to
try other zspages.

Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
---
 mm/zsmalloc.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index fda7177..310c7b0 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -765,6 +765,9 @@ static int get_pages_per_zspage(int class_size)
 		if (usedpc > max_usedpc) {
 			max_usedpc = usedpc;
 			max_usedpc_order = i;
+
+			if (max_usedpc == 100)
+				break;
 		}
 	}
 
-- 
1.7.9.5

Re: [PATCH] mm/zsmalloc: avoid unnecessary iteration in get_pages_per_zspage()

[Sergey Senozhatsky]

On (05/05/16 13:17), Ganesh Mahendran wrote:
> if we find a zspage with usage == 100%, there is no need to
> try other zspages.

Hello,

well... we iterate there from 0 to 1<<2, which is not awfully
a lot to break it in the middle, and we do this only when we
initialize a new pool (for every size class).

the check is
 - true   15 times
 - false  492 times

so it _sort of_ feels like this new if-condition doesn't
buy us a lot, and most of the time it just sits there with
no particular gain. let's hear from Minchan.

	-ss

> Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Nitin Gupta <ngupta@vflare.org>
> Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
> ---
>  mm/zsmalloc.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index fda7177..310c7b0 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -765,6 +765,9 @@ static int get_pages_per_zspage(int class_size)
>  		if (usedpc > max_usedpc) {
>  			max_usedpc = usedpc;
>  			max_usedpc_order = i;
> +
> +			if (max_usedpc == 100)
> +				break;
>  		}
>  	}
>  
> -- 
> 1.7.9.5
> 

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Re: [PATCH] mm/zsmalloc: avoid unnecessary iteration in get_pages_per_zspage()

[Minchan Kim]

On Thu, May 05, 2016 at 07:03:29PM +0900, Sergey Senozhatsky wrote:
> On (05/05/16 13:17), Ganesh Mahendran wrote:
> > if we find a zspage with usage == 100%, there is no need to
> > try other zspages.
> 
> Hello,
> 
> well... we iterate there from 0 to 1<<2, which is not awfully
> a lot to break it in the middle, and we do this only when we
> initialize a new pool (for every size class).
> 
> the check is
>  - true   15 times
>  - false  492 times

Thanks for the data, Sergey!

> 
> so it _sort of_ feels like this new if-condition doesn't
> buy us a lot, and most of the time it just sits there with
> no particular gain. let's hear from Minchan.
> 

I agree with Sergey.
First of al, I appreciates your patch, Ganesh! But as Sergey pointed
out, I don't see why it improves current zsmalloc.
If you want to merge strongly, please convince me with more detail
reason.

Thanks.


> 	-ss
> 
> > Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
> > Cc: Minchan Kim <minchan@kernel.org>
> > Cc: Nitin Gupta <ngupta@vflare.org>
> > Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
> > ---
> >  mm/zsmalloc.c |    3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > index fda7177..310c7b0 100644
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -765,6 +765,9 @@ static int get_pages_per_zspage(int class_size)
> >  		if (usedpc > max_usedpc) {
> >  			max_usedpc = usedpc;
> >  			max_usedpc_order = i;
> > +
> > +			if (max_usedpc == 100)
> > +				break;
> >  		}
> >  	}
> >  
> > -- 
> > 1.7.9.5
> > 

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Re: [PATCH] mm/zsmalloc: avoid unnecessary iteration in get_pages_per_zspage()

[Ganesh Mahendran]

Hi, Minchan:

2016-05-06 11:09 GMT+08:00 Minchan Kim <minchan@kernel.org>:
> On Thu, May 05, 2016 at 07:03:29PM +0900, Sergey Senozhatsky wrote:
>> On (05/05/16 13:17), Ganesh Mahendran wrote:
>> > if we find a zspage with usage == 100%, there is no need to
>> > try other zspages.
>>
>> Hello,
>>
>> well... we iterate there from 0 to 1<<2, which is not awfully
>> a lot to break it in the middle, and we do this only when we
>> initialize a new pool (for every size class).
>>
>> the check is
>>  - true   15 times
>>  - false  492 times
>
> Thanks for the data, Sergey!
>
>>
>> so it _sort of_ feels like this new if-condition doesn't
>> buy us a lot, and most of the time it just sits there with
>> no particular gain. let's hear from Minchan.
>>
>
> I agree with Sergey.
> First of al, I appreciates your patch, Ganesh! But as Sergey pointed
> out, I don't see why it improves current zsmalloc.

This patch does not obviously improve zsmalloc.
It just reduces unnecessary code path.

>From data provided by Sergey, 15 * (4 -  1) = 45 times loop will be avoided.
So 45 times of below caculation will be reduced:
---
zspage_size = i * PAGE_SIZE;
waste = zspage_size % class_size;
usedpc = (zspage_size - waste) * 100 / zspage_size;

if (usedpc > max_usedpc) {
---

Thanks.

> If you want to merge strongly, please convince me with more detail
> reason.
>
> Thanks.
>
>
>>       -ss
>>
>> > Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
>> > Cc: Minchan Kim <minchan@kernel.org>
>> > Cc: Nitin Gupta <ngupta@vflare.org>
>> > Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
>> > ---
>> >  mm/zsmalloc.c |    3 +++
>> >  1 file changed, 3 insertions(+)
>> >
>> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
>> > index fda7177..310c7b0 100644
>> > --- a/mm/zsmalloc.c
>> > +++ b/mm/zsmalloc.c
>> > @@ -765,6 +765,9 @@ static int get_pages_per_zspage(int class_size)
>> >             if (usedpc > max_usedpc) {
>> >                     max_usedpc = usedpc;
>> >                     max_usedpc_order = i;
>> > +
>> > +                   if (max_usedpc == 100)
>> > +                           break;
>> >             }
>> >     }
>> >
>> > --
>> > 1.7.9.5
>> >

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Re: [PATCH] mm/zsmalloc: avoid unnecessary iteration in get_pages_per_zspage()

[Minchan Kim]

Hi Ganesh,

On Fri, May 06, 2016 at 12:25:18PM +0800, Ganesh Mahendran wrote:
> Hi, Minchan:
> 
> 2016-05-06 11:09 GMT+08:00 Minchan Kim <minchan@kernel.org>:
> > On Thu, May 05, 2016 at 07:03:29PM +0900, Sergey Senozhatsky wrote:
> >> On (05/05/16 13:17), Ganesh Mahendran wrote:
> >> > if we find a zspage with usage == 100%, there is no need to
> >> > try other zspages.
> >>
> >> Hello,
> >>
> >> well... we iterate there from 0 to 1<<2, which is not awfully
> >> a lot to break it in the middle, and we do this only when we
> >> initialize a new pool (for every size class).
> >>
> >> the check is
> >>  - true   15 times
> >>  - false  492 times
> >
> > Thanks for the data, Sergey!
> >
> >>
> >> so it _sort of_ feels like this new if-condition doesn't
> >> buy us a lot, and most of the time it just sits there with
> >> no particular gain. let's hear from Minchan.
> >>
> >
> > I agree with Sergey.
> > First of al, I appreciates your patch, Ganesh! But as Sergey pointed
> > out, I don't see why it improves current zsmalloc.
> 
> This patch does not obviously improve zsmalloc.
> It just reduces unnecessary code path.
> 
> From data provided by Sergey, 15 * (4 -  1) = 45 times loop will be avoided.
> So 45 times of below caculation will be reduced:
> ---
> zspage_size = i * PAGE_SIZE;
> waste = zspage_size % class_size;
> usedpc = (zspage_size - waste) * 100 / zspage_size;
> 
> if (usedpc > max_usedpc) {

As well, it bloats code side without much gain. I don't think
it's worth to do until someone really has trouble with slow
zs_create_pool performance.

add/remove: 0/0 grow/shrink: 1/0 up/down: 15/0 (15)
function                                     old     new   delta
zs_create_pool                               960     975     +15


> ---
> 
> Thanks.
> 
> > If you want to merge strongly, please convince me with more detail
> > reason.
> >
> > Thanks.
> >
> >
> >>       -ss
> >>
> >> > Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
> >> > Cc: Minchan Kim <minchan@kernel.org>
> >> > Cc: Nitin Gupta <ngupta@vflare.org>
> >> > Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
> >> > ---
> >> >  mm/zsmalloc.c |    3 +++
> >> >  1 file changed, 3 insertions(+)
> >> >
> >> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> >> > index fda7177..310c7b0 100644
> >> > --- a/mm/zsmalloc.c
> >> > +++ b/mm/zsmalloc.c
> >> > @@ -765,6 +765,9 @@ static int get_pages_per_zspage(int class_size)
> >> >             if (usedpc > max_usedpc) {
> >> >                     max_usedpc = usedpc;
> >> >                     max_usedpc_order = i;
> >> > +
> >> > +                   if (max_usedpc == 100)
> >> > +                           break;
> >> >             }
> >> >     }
> >> >
> >> > --
> >> > 1.7.9.5
> >> >

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Re: [PATCH] mm/zsmalloc: avoid unnecessary iteration in get_pages_per_zspage()

[Sergey Senozhatsky]

On (05/06/16 12:25), Ganesh Mahendran wrote:
[..]
> > I agree with Sergey.
> > First of al, I appreciates your patch, Ganesh! But as Sergey pointed
> > out, I don't see why it improves current zsmalloc.
> 
> This patch does not obviously improve zsmalloc.
> It just reduces unnecessary code path.
> 
> From data provided by Sergey, 15 * (4 -  1) = 45 times loop will be avoided.
> So 45 times of below caculation will be reduced:
> ---
> zspage_size = i * PAGE_SIZE;
> waste = zspage_size % class_size;
> usedpc = (zspage_size - waste) * 100 / zspage_size;
> 
> if (usedpc > max_usedpc) {
> ---

Hello,

I kinda believe we end up doing more work (instruction-count-wise),
actually. it adds 495 `cmp' for false case + 15 `cmp je' for true
case to eliminate 15 `mov cltd idiv mov sub imul cltd idiv cmp' *.

and it's not 45 iterations that we are getting rid of, but around 31:
not every class reaches it's ideal 100% ratio on the first iteration.
so, no, sorry, I don't think the patch really does what we want.



* by the way, we don't even need `cltd' in those calculations. the
reason why gcc puts cltd is because ZS_MAX_PAGES_PER_ZSPAGE has the
'wrong' data type. the patch to correct it is below (not a formal
patch).

** well, we force gcc to generate `worse' code in several more places.
for example, there is no need for `obj_idx' and `obj_offset' to be
`unsigned long', it can easly (and probably must) be `unsigned int',
or simply `int'. that can save some instructions in very-very hot paths:

add/remove: 0/0 grow/shrink: 1/6 up/down: 1/-27 (-26)
function                                     old     new   delta
obj_free                                     234     235      +1
obj_to_location                               45      44      -1
obj_malloc                                   234     233      -1
zs_malloc                                    817     815      -2
obj_idx_to_offset                             32      28      -4
zs_unmap_object                              556     551      -5
zs_compact                                  1611    1597     -14

I can cook a trivial patch later.

/*
 * on x86_64, gcc 6.1. no idea what does the picture look like on ARM32.
 * but smells like these two patches combined can make CPU a little less
 * busy.
 */

=====================================================================

ZS_MAX_PAGES_PER_ZSPAGE defined as 'unsigned long' which forces
the compiler to generate unneeded signed extension instructions
`cltd' in several places. for instance:

     711:       44 89 d0                mov    %r10d,%eax
     714:       99                      cltd
     715:       41 f7 fe                idiv   %r14d
     718:       44 89 d0                mov    %r10d,%eax
     71b:       29 d0                   sub    %edx,%eax
     71d:       6b c0 64                imul   $0x64,%eax,%eax
     720:       99                      cltd
     721:       41 f7 fa                idiv   %r10d

there is no reason to do this and ZS_MAX_PAGES_PER_ZSPAGE can
simply be 'int'.

the patch reduces the code size, a bit:

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-25 (-25)
function                                     old     new   delta
zs_malloc                                    842     817     -25

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 mm/zsmalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index f9b58d1..1c28e0f6 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -78,7 +78,7 @@
  * pages. ZS_MAX_ZSPAGE_ORDER defines upper limit on N.
  */
 #define ZS_MAX_ZSPAGE_ORDER 2
-#define ZS_MAX_PAGES_PER_ZSPAGE (_AC(1, UL) << ZS_MAX_ZSPAGE_ORDER)
+#define ZS_MAX_PAGES_PER_ZSPAGE (1 << ZS_MAX_ZSPAGE_ORDER)
 
 #define ZS_HANDLE_SIZE (sizeof(unsigned long))
 
-- 
2.8.2

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Re: [PATCH] mm/zsmalloc: avoid unnecessary iteration in get_pages_per_zspage()

[Sergey Senozhatsky]

On (05/06/16 18:08), Sergey Senozhatsky wrote:
[..]
> and it's not 45 iterations that we are getting rid of, but around 31:
> not every class reaches it's ideal 100% ratio on the first iteration.
> so, no, sorry, I don't think the patch really does what we want.


to be clear, what I meant was:

  495 `cmp' + 15 `cmp je'                         IN
  31 `mov cltd idiv mov sub imul cltd idiv cmp'   OUT

IN > OUT.


CORRECTION here:

> * by the way, we don't even need `cltd' in those calculations. the
> reason why gcc puts cltd is because ZS_MAX_PAGES_PER_ZSPAGE has the
> 'wrong' data type. the patch to correct it is below (not a formal
> patch).

no, we need cltd there. but ZS_MAX_PAGES_PER_ZSPAGE also affects
ZS_MIN_ALLOC_SIZE, which is used in several places, like
get_size_class_index(). that's why ZS_MAX_PAGES_PER_ZSPAGE data
type change `improves' zs_malloc().

	-ss

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Re: [PATCH] mm/zsmalloc: avoid unnecessary iteration in get_pages_per_zspage()

[Minchan Kim]

On Fri, May 06, 2016 at 06:33:42PM +0900, Sergey Senozhatsky wrote:
> On (05/06/16 18:08), Sergey Senozhatsky wrote:
> [..]
> > and it's not 45 iterations that we are getting rid of, but around 31:
> > not every class reaches it's ideal 100% ratio on the first iteration.
> > so, no, sorry, I don't think the patch really does what we want.
> 
> 
> to be clear, what I meant was:
> 
>   495 `cmp' + 15 `cmp je'                         IN
>   31 `mov cltd idiv mov sub imul cltd idiv cmp'   OUT
> 
> IN > OUT.
> 
> 
> CORRECTION here:
> 
> > * by the way, we don't even need `cltd' in those calculations. the
> > reason why gcc puts cltd is because ZS_MAX_PAGES_PER_ZSPAGE has the
> > 'wrong' data type. the patch to correct it is below (not a formal
> > patch).
> 
> no, we need cltd there. but ZS_MAX_PAGES_PER_ZSPAGE also affects
> ZS_MIN_ALLOC_SIZE, which is used in several places, like
> get_size_class_index(). that's why ZS_MAX_PAGES_PER_ZSPAGE data
> type change `improves' zs_malloc().

Why not if such simple improves zsmalloc? :)
Please send a patch.

Thanks a lot, Sergey!

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Re: [PATCH] mm/zsmalloc: avoid unnecessary iteration in get_pages_per_zspage()

[Sergey Senozhatsky]

On (05/09/16 14:01), Minchan Kim wrote:
[..]
> > no, we need cltd there. but ZS_MAX_PAGES_PER_ZSPAGE also affects
> > ZS_MIN_ALLOC_SIZE, which is used in several places, like
> > get_size_class_index(). that's why ZS_MAX_PAGES_PER_ZSPAGE data
> > type change `improves' zs_malloc().
> 
> Why not if such simple improves zsmalloc? :)
> Please send a patch.
> 
> Thanks a lot, Sergey!

Hello Minchan,

sorry for long reply, I decided to investigate it a bit further.
with this patch, gcc 6.1 -O2 generates "+13" instructions more,
-Os "-25" instructions less. this +13 ins case is a no-no-no.

	-ss

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