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> > > ---
