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 kqQhKHKk 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Andrew Morton writes: > On Mon, 15 Dec 2025 22:49:25 -0800 Ankur Arora wrote: > >> >> [#] Notice that we perform much better with preempt=full|lazy. As >> >> mentioned above, preemptible models not needing explicit invocations >> >> of cond_resched() allow clearing of the full extent (1GB) as a >> >> single unit. >> >> In comparison the maximum extent used for preempt=none|voluntary is >> >> PROCESS_PAGES_NON_PREEMPT_BATCH (8MB). >> >> >> >> The larger extent allows the processor to elide cacheline >> >> allocation (on Milan the threshold is LLC-size=32MB.) >> > >> > It is this? >> >> Yeah I think so. For size >= 32MB, the microcoder can really just elide >> cacheline allocation, and with the foreknowledge of the extent can perhaps >> optimize on cache coherence traffic (this last one is my speculation). >> >> On cacheline allocation elision, compare the L1-dcache-load in the two versions >> below: >> >> pg-sz=1GB: >> - 9,250,034,512 cycles # 2.418 GHz ( +- 0.43% ) (46.16%) >> - 544,878,976 instructions # 0.06 insn per cycle >> - 2,331,332,516 L1-dcache-loads # 609.471 M/sec ( +- 0.03% ) (46.16%) >> - 1,075,122,960 L1-dcache-load-misses # 46.12% of all L1-dcache accesses ( +- 0.01% ) (46.15%) >> >> + 3,688,681,006 cycles # 2.420 GHz ( +- 3.48% ) (46.01%) >> + 10,979,121 instructions # 0.00 insn per cycle >> + 31,829,258 L1-dcache-loads # 20.881 M/sec ( +- 4.92% ) (46.34%) >> + 13,677,295 L1-dcache-load-misses # 42.97% of all L1-dcache accesses ( +- 6.15% ) (46.32%) >> > > That says L1 d-cache loads went from 600 million/sec down to 20 > million/sec when using 32MB chunks? Sorry, should have mentioned that that run was with preempt=full/lazy. For those the chunk size is the whole page (GB page in that case). The context for 32MB was that that's the LLC-size for these systems. And, from observed behaviour the cacheline allocation elision optimization only happens when the chunk size used is larger than that. > Do you know what happens to preemption latency if you increase that > chunk size from 8MB to 32MB? So, I gathered some numbers on a Zen4/Genoa system. The ones above are from Zen3/Milan. region-sz=64GB, loop-count=3 (total region-size=3*64GB): Bandwidth L1-dcache-loads pg-sz=2MB, batch-sz= 8MB 25.10 GB/s 6,745,859,855 # 2.00 L1-dcache-loads/64B # pg-sz=2MB for context pg-sz=1GB, batch-sz= 8MB 26.88 GB/s 6,469,900,728 # 2.00 L1-dcache-loads/64B pg-sz=1GB, batch-sz=32MB 38.69 GB/s 2,559,249,546 # 0.79 L1-dcache-loads/64B pg-sz=1GB, batch-sz=64MB 46.91 GB/s 919,539,544 # 0.28 L1-dcache-loads/64B pg-sz=1GB, batch-sz= 1GB 58.68 GB/s 79,458,439 # 0.024 L1-dcache-loads/64B All of these are for preempt=none, and with boost=0. (With boost=1 the BW increases by ~25%.) So, I wasn't quite right about the LLC-size=32MB being the threshold for this optimization. There is a change in behaviour at that point but it does improve beyond that. (Ideally this threshold would be a processor MSR. That way we could use this for 2MB pages as well. Oh well.) > At 42GB/sec, 32MB will take less than a > millisecond, yes? I'm not aware of us really having any latency > targets in these preemption modes, but 1 millisecond sounds pretty > good. Agreed. The only complaint threshold I see is 100ms (default value of sysctl_resched_latency_warn_ms) which is pretty far from ~1ms. And having a threshold of 32MB might benefit other applications since we won't be discarding their cachelines in favour of filling up the cache with zeroes. I think the only problem cases might be slow uarchs and workloads where the memory bus is saturated which might dilate the preemption latency. And, even if the operation takes say ~20ms, that should still leave us with a reasonably large margin. (And, any latency senstive users are probably not running with preempt=none/voluntary.) -- ankur