Messages in this thread | | | Date | Tue, 15 Mar 2022 17:23:00 +0100 | From | Ahmad Fatoum <> | Subject | Possible performance regression with CONFIG_SQUASHFS_DECOMP_SINGLE |
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Hello,
This an issue we had with v5.15 that we have since successfully worked around. I am reporting it here as a pointer in case someone else runs into this and as a heads up that there seems to be an underlying performance regression, so here it goes:
We have an i.MX8MM (4x Cortex-A53) system with squashfs on eMMC as a root file system. The system originally ran NXP's "imx_5.4.24_2.1.0" which has about 5000 patches on top of upstream v5.4.24 including PREEMPT_RT.
The system was updated to mainline Linux + PREEMPT_RT and boot time suffered considerably growing from 40s with vendor kernel to 1m20s with mainline-based kernel.
The slowdown on mainline was reproducible for all scheduling models (with or without PREEMPT_RT) except for PREEMPT_NONE, which was back at 40s.
The services most impacted by the slowdown were C++ applications with many shared libraries dynamically loaded from the rootfs.
Looking through the original kernel configuration we found that it has CONFIG_SQUASHFS_DECOMP_SINGLE=y and CONFIG_SQUASHFS_FILE_CACHE=y.
Once changed to CONFIG_SQUASHFS_FILE_DIRECT=y and CONFIG_SQUASHFS_DECOMP_MULTI_PERCPU=y, we were below 40s as we want.
That's clearly the preferred configuration and it resolves our problem It doesn't solve the underlying issue though:
- CONFIG_PREEMPT_VOLUNTARY performs much worse than CONFIG_PREEMPT_NONE for some workloads when CONFIG_SQUASHFS_DECOMP_SINGLE=y
- And this might not have been the case with v5.4. Unfortunately we can't bisect, because there wasn't enough i.MX8MM support mainline back then to boot the system. Earliest mainline-based kernel we reproduced this on was v5.11.
TL;DR: Check if CONFIG_SQUASHFS_DECOMP_MULTI_PERCPU=y in your configuration
Cheers, Ahmad
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