Re: [RFC PATCH] mm, slob: Rewrite SLOB using segregated free list

[Hyeonggon Yoo]

Hmm.. I think I need to clarify my intention.

I'm not saying this should be merged or we should put effort to make
SLOB into lightweight SLOB. I just rewrote it just for fun.
I wanted to know how small a segregated free list allocator can be.

And when I rewrote it, I wondered who is users of SLOB
and where SLOB should be used.

I think SLOB was useful when there was only SLAB and there was no SLUB,
but I wonder where SLOB should be used now.

When I compared SLOB and SLUB without cpu partials,
That made 300kB of difference in Slab memory.

Then Is SLOB used where 300kB of difference is so important?
But I think we need at least 16MB of RAM to run linux.

So I'm not saying we need to turn SLOB into lightweight SLUB,
but wanted to talk about the questions:

> > But after rewriting, I thought I need to discuss what SLOB is for.
> > According to Matthew, SLOB is for small machines whose
> > memory is 1~16 MB.
> > 
> > I wonder adding 48kB on SLOB memory for speed/lower latency
> > is worth or harmful.
> > 
> > So.. questions in my head now:
> >     - Who is users of SLOB?
> >     - Is it harmful to add some kilobytes of memory into SLOB?
> >     - Is it really possible to run linux under 10MB of RAM?
> >     	(I failed with tinyconfig.)
> >     - What is the boundary to make decision between SLOB and SLUB?

On Thu, Oct 21, 2021 at 10:46:46AM +0200, Vlastimil Babka wrote:
> On 10/20/21 15:55, Hyeonggon Yoo wrote:
> > Hello linux-mm, I rewrote SLOB using segregated free list,
> > to understand SLOB and SLUB more. It uses more kilobytes
> > of memory (48kB on 32bit tinyconfig) and became 9~10x faster.
> > 
> > But after rewriting, I thought I need to discuss what SLOB is for.
> > According to Matthew, SLOB is for small machines whose
> > memory is 1~16 MB.
> > 
> > I wonder adding 48kB on SLOB memory for speed/lower latency
> > is worth or harmful.
> > 
> > So.. questions in my head now:
> >     - Who is users of SLOB?
> >     - Is it harmful to add some kilobytes of memory into SLOB?
> >     - Is it really possible to run linux under 10MB of RAM?
> >     	(I failed with tinyconfig.)
> >     - What is the boundary to make decision between SLOB and SLUB?
> > 
> > Anyway, below is my work.
> > Any comments/opinions will be appreciated!
> > 
> > SLOB uses sequential fit method. the advantages of this method
> > is the fact that it is simple and does not have complex metadata.
> > 
> > But big downside of sequential fit method is its high latency
> > in allocation/deallocation and fast fragmentation.
> > 
> > High latency comes from iterating pages and also iterating objects
> > in the page to find suitable free object. And fragmentation easily
> > happens because objects of difference size is allocated in same page.
> > 
> > This patch tries to minimize both its latency and fragmentation by
> > re-implmenting SLOB using segregated free list method and adding
> > support for slab merging. it looks like lightweight SLUB but more
> > compact than SLUB.
> 
> My immediate reaction is that we probably don't want to turn SLOB into
> lightweight SLUB. SLOB choses the tradeoff of low memory usage over speed
> and shifting it towards more speed kinda defeats this purpose. Also it's a
> major rewrite, so without a very clear motivation there will be resistance
> to that.
>

Yes, I agree that SLOB is for memory efficiency, not a performance.
That's why I said:
> > I wonder adding 48kB on SLOB memory for speed/lower latency
> > is worth or harmful.

But on the contrary, I wonder when SLOB is useful than SLUB.
is it for really tiny linux systems that has under 1M of RAM?
But can linux be that small?

> SLUB itself could be probably tuned to less memory overhead if needed. Most
> of the debug options effectively disable percpu slabs, we could add a mode
> that disables them without the rest of the debugging overhead. Allocation
> order can be lowered (although some object sizes might benefit from less
> fragmentation with a higher order).

Yes, that's what I was curious about. As SLUB is not that big,
I wonder where SLOB is useful.

> > One notable difference is after this patch SLOB uses kmalloc_caches
> > like SL[AU]B.
> > 
> > Below is performance impacts of this patch.
> > 
> > Memory usage was measured on 32 bit + tinyconfig + slab merging.
> > 
> > Before:
> >     MemTotal:          29668 kB
> >     MemFree:           19364 kB
> >     MemAvailable:      18396 kB
> >     Slab:                668 kB
> > 
> > After:
> >     MemTotal:          29668 kB
> >     MemFree:           19420 kB
> >     MemAvailable:      18452 kB
> >     Slab:                716 kB
> > 
> > This patch adds about 48 kB after boot.
> > 
> > hackbench was measured on 64 bit typical buildroot configuration.
> > After this patch it's 9~10x faster than before.
> > 
> > Before:
> >     memory usage:
> >         after boot:
> >             Slab:               7908 kB
> >         after hackbench:
> >             Slab:               8544 kB
> > 
> >     Time: 189.947
> >     Performance counter stats for 'hackbench -g 4 -l 10000':
> >          379413.20 msec cpu-clock                 #    1.997 CPUs utilized
> >            8818226      context-switches          #   23.242 K/sec
> >             375186      cpu-migrations            #  988.859 /sec
> >               3954      page-faults               #   10.421 /sec
> >       269923095290      cycles                    #    0.711 GHz
> >       212341582012      instructions              #    0.79  insn per cycle
> >         2361087153      branch-misses
> >        58222839688      cache-references          #  153.455 M/sec
> >         6786521959      cache-misses              #   11.656 % of all cache refs
> > 
> >      190.002062273 seconds time elapsed
> > 
> >        3.486150000 seconds user
> >      375.599495000 seconds sys
> > 
> > After:
> >     memory usage:
> >        after boot:
> >            Slab:               7560 kB
> >         after hackbench:
> >            Slab:               7836 kB
> 
> Interesting that the memory usage in this test is actually lower with your
> patch.

I didn't mention that because if we have enough memory,
I think we have no reason to use SLOB. (why not use SLUB?)
I thought memory usage on small machine is important.

> 
> > hackbench:
> >     Time: 20.780
> >     Performance counter stats for 'hackbench -g 4 -l 10000':
> >           41509.79 msec cpu-clock                 #    1.996 CPUs utilized
> >             630032      context-switches          #   15.178 K/sec
> >               8287      cpu-migrations            #  199.640 /sec
> >               4036      page-faults               #   97.230 /sec
> >        57477161020      cycles                    #    1.385 GHz
> >        62775453932      instructions              #    1.09  insn per cycle
> >          164902523      branch-misses
> >        22559952993      cache-references          #  543.485 M/sec
> >          832404011      cache-misses              #    3.690 % of all cache refs
> > 
> >       20.791893590 seconds time elapsed
> > 
> >        1.423282000 seconds user
> >       40.072449000 seconds sys
> 
> That's significant, but also hackbench is kind of worst case test, so in
> practice the benefit won't be that prominent.

> 
> > Signed-off-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> > ---