On 10/17/24 01:19, Cristian Prundeanu wrote:
This patchset disables the scheduler features PLACE_LAG and RUN_TO_PARITY and moves them to sysctl. Replacing CFS with the EEVDF scheduler in kernel 6.6 introduced significant performance degradation in multiple database-oriented workloads. This degradation manifests in all kernel versions using EEVDF, across multiple Linux distributions, hardware architectures (x86_64, aarm64, amd64), and CPU generations. For example, running mysql+hammerdb results in a 12-17% throughput reduction and 12-18% latency increase compared to kernel 6.5 (using default scheduler settings everywhere). The magnitude of this performance impact is comparable to the average performance difference of a CPU generation over its predecessor. Testing combinations of available scheduler features showed that the largest improvement (short of disabling all EEVDF features) came from disabling both PLACE_LAG and RUN_TO_PARITY: Kernel | default | NO_PLACE_LAG and aarm64 | config | NO_RUN_TO_PARITY ---------+----------+----------------- 6.5 | baseline | N/A 6.6 | -13.2% | -6.8% 6.7 | -13.1% | -6.0% 6.8 | -12.3% | -6.5% 6.9 | -12.7% | -6.9% 6.10 | -13.5% | -5.8% 6.11 | -12.6% | -5.8% 6.12-rc2 | -12.2% | -8.9% ---------+----------+----------------- Kernel | default | NO_PLACE_LAG and x86_64 | config | NO_RUN_TO_PARITY ---------+----------+----------------- 6.5 | baseline | N/A 6.6 | -16.8% | -10.8% 6.7 | -16.4% | -9.9% 6.8 | -17.2% | -9.5% 6.9 | -17.4% | -9.7% 6.10 | -16.5% | -9.0% 6.11 | -15.0% | -8.5% 6.12-rc2 | -12.7% | -10.9% ---------+----------+----------------- While the long term approach is debugging and fixing the scheduler behavior, algorithm changes to address performance issues of this nature are specialized (and likely prolonged or open-ended) research. Until a change is identified which fixes the performance degradation, in the interest of a better out-of-the-box performance: (1) disable these features by default, and (2) expose these values in sysctl instead of debugfs, so they can be more easily persisted across reboots. Cristian Prundeanu (2): sched: Disable PLACE_LAG and RUN_TO_PARITY sched: Move PLACE_LAG and RUN_TO_PARITY to sysctl include/linux/sched/sysctl.h | 8 ++++++++ kernel/sched/core.c | 13 +++++++++++++ kernel/sched/fair.c | 5 +++-- kernel/sched/features.h | 10 ---------- kernel/sysctl.c | 20 ++++++++++++++++++++ 5 files changed, 44 insertions(+), 12 deletions(-)
Hi Cristian,This is a confirmation that we are also seeing a 9% performance regression with the TPCC benchmark after v6.6-rc1. We narrowed down the regression was caused due to commit:
86bfbb7ce4f6 ("sched/fair: Add lag based placement")
This regression was reported via this thread:
https://lore.kernel.org/lkml/1c447727-92ed-416c-bca1-a7ca0974f0df@xxxxxxxxxx/
Phil Auld suggested to try turning off the PLACE_LAG sched feature. We
tested with NO_PLACE_LAG and can confirm it brought back 5% of the
performance loss. We do not yet know what effect NO_PLACE_LAG will have
on other benchmarks, but it indeed helps TPCC.
Thanks for the work to move PLACE_LAG and RUN_TO_PARITY to sysctl! Joe
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