segregated list + slab merging is much better than original SLOB

[Hyeonggon Yoo]

On Sun, Oct 17, 2021 at 04:28:52AM +0000, Hyeonggon Yoo wrote:
> I've been reading SLUB/SLOB code for a while. SLUB recently became
> real time compatible by reducing its locking area.
> 
> for now, SLUB is the only slab allocator for PREEMPT_RT because
> it works better than SLAB on RT and SLOB uses non-deterministic method,
> sequential fit.
> 
> But memory usage of SLUB is too high for systems with low memory.
> So In my local repository I made SLOB to use segregated free list
> method, which is more more deterministic, to provide bounded latency.
> 
> This can be done by managing list of partial pages globally
> for every power of two sizes (8, 16, 32, ..., PAGE_SIZE) per NUMA nodes.
> minimal allocation size is size of pointers to keep pointer of next free object
> like SLUB.
> 
> By making objects in same page to have same size, there's no
> need to iterate free blocks in a page. (Also iterating pages isn't needed)
> 
> Some cleanups and more tests (especially with NUMA/RT configs) needed,
> but want to hear your opinion about the idea. Did not test on RT yet.
> 
> Below is result of benchmarks and memory usage. (on !RT)
> with 13% increase in memory usage, it's nine times faster and
> bounded fragmentation, and importantly provides predictable execution time.
> 

Hello linux-mm, I improved it and it uses lower memory
and 9x~13x faster than original SLOB. it shows much less fragmentation
after hackbench.

Rather than managing global freelist that has power of 2 sizes,
I made a kmem_cache to manage its own freelist (for each NUMA nodes) and
Added support for slab merging. So It quite looks like a lightweight SLUB now.

I'll send rfc patch after some testing and code cleaning.

I think it is more RT-friendly becuase it's uses more deterministic
algorithm (But lock is still shared among cpus). Any opinions for RT?

current SLOB:
    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

SLOB with segregated list + slab merging:
    memory usage:
       after boot:
           Slab:               7560 kB
        after hackbench:
           Slab:               7836 kB        

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
-
Thanks,
Hyeonggon