Messages in this thread | | | Date | Mon, 27 Mar 2023 09:20:44 -0400 | Subject | Re: rq lock contention due to commit af7f588d8f73 | From | Mathieu Desnoyers <> |
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On 2023-03-27 04:05, Aaron Lu wrote: > Hi Mathieu, > > I was doing some optimization work[1] for kernel scheduler using a > database workload: sysbench+postgres and before I submit my work, I > rebased my patch on top of latest v6.3-rc kernels to see if everything > still works expected and then I found rq's lock became very heavily > contended as compared to v6.2 based kernels. > > Using the above mentioned workload, before commit af7f588d8f73("sched: > Introduce per-memory-map concurrency ID"), the profile looked like: > > 7.30% 0.71% [kernel.vmlinux] [k] __schedule > 0.03% 0.03% [kernel.vmlinux] [k] native_queued_spin_lock_slowpath > > After that commit: > > 49.01% 0.87% [kernel.vmlinux] [k] __schedule > 43.20% 43.18% [kernel.vmlinux] [k] native_queued_spin_lock_slowpath > > The above profile was captured with sysbench's nr_threads set to 56; if > I used more thread number, the contention would be more severe on that > 2sockets/112core/224cpu Intel Sapphire Rapids server. > > The docker image I used to do optimization work is not available outside > but I managed to reproduce this problem using only publicaly available > stuffs, here it goes: > 1 docker pull postgres > 2 sudo docker run --rm --name postgres-instance -e POSTGRES_PASSWORD=mypass -e POSTGRES_USER=sbtest -d postgres -c shared_buffers=80MB -c max_connections=250 > 3 go inside the container > sudo docker exec -it $the_just_started_container_id bash > 4 install sysbench inside container > sudo apt update and sudo apt install sysbench > 5 prepare > root@container:/# sysbench --db-driver=pgsql --pgsql-user=sbtest --pgsql_password=mypass --pgsql-db=sbtest --pgsql-port=5432 --tables=16 --table-size=10000 --threads=56 --time=60 --report-interval=2 /usr/share/sysbench/oltp_read_only.lua prepare > 6 run > root@container:/# sysbench --db-driver=pgsql --pgsql-user=sbtest --pgsql_password=mypass --pgsql-db=sbtest --pgsql-port=5432 --tables=16 --table-size=10000 --threads=56 --time=60 --report-interval=2 /usr/share/sysbench/oltp_read_only.lua run > > Let it warm up a little bit and after 10-20s you can do profile and see > the increased rq lock contention. You may need a machine that has at > least 56 cpus to see this, I didn't try on other machines. > > Feel free to let me know if you need any other info.
While I setup my dev machine with this reproducer, here are a few questions to help figure out the context:
I understand that pgsql is a multi-process database. Is it strictly single-threaded per-process, or does each process have more than one thread ?
I understand that your workload is scheduling between threads which belong to different processes. Are there more heavily active threads than there are scheduler runqueues (CPUs) on your machine ?
When I developed the mm_cid feature, I originally implemented two additional optimizations:
Additional optimizations can be done if the spin locks added when context switching between threads belonging to different memory maps end up being a performance bottleneck. Those are left out of this patch though. A performance impact would have to be clearly demonstrated to justify the added complexity.
I suspect that your workload demonstrates the need for at least one of those optimizations. I just wonder if we are in a purely single-threaded scenario for each process, or if each process has many threads.
Thanks,
Mathieu
> > [1]: https://lore.kernel.org/lkml/20230327053955.GA570404@ziqianlu-desk2/ > > Best wishes, > Aaron
-- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com
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