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malwarescore=0 mlxlogscore=999 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2409260000 definitions=main-2411140158 X-Proofpoint-ORIG-GUID: bEOSYXTvrwA7vkXUbJqZPJ7gl_PCCWE9 X-Proofpoint-GUID: bEOSYXTvrwA7vkXUbJqZPJ7gl_PCCWE9 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@oracle.com/ 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