linux-kernel - x86 : Slow read/write from tmpfs after major kernel upgrades

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Message-ID: <LO4P265MB739293B94D911AE33A496EDFEBE9A@LO4P265MB7392.GBRP265.PROD.OUTLOOK.COM>
Date: Thu, 16 Oct 2025 19:37:36 +0000
From: Elison Niven <Elison.Niven@...hos.com>
To: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: x86 : Slow read/write from tmpfs after major kernel upgrades

We have noticed severe degradation in read/write operations from tmpfs while upgrading from kernel 4.x.y to kernel 6.6.x on x86 hardware.

To demonstrate this, I built linux kernel with make defconfig ARCH=x86_64 and booted it with a minimal busybox based initramfs.

# Mount tmpfs at /tmp :
mount -t tmpfs tmpfs /tmp

# Write a 200KB file using dd with 512 block size :
dd if=/dev/zero of=/tmp/200k bs=512 count=400

# Create a 64 MB file in /tmp/ :
dd if=/dev/zero of=/tmp/test bs=4k count=16k

# Read the file back
dd if=/tmp/test of=/dev/zero bs=4k

The average write speeds for writing a 200 KB file with bs=512 measured on different kernel versions were :

Linux version 4.20.17 : 500 MB/s
Linux version 5.4.300 : 460 MB/s
Linux version 5.19.17 : 380 MB/s
Linux version 6.6.108 : 240 MB/s
Linux version 6.17.2 : 277 MB/s

The average read speeds for reading a 64 MB file with bs=4k measured on different kernel versions were :

Linux version 4.20.17 : 4.7 GB/s
Linux version 5.4.300 : 4.5 GB/s
Linux version 5.19.17 :  4.3 GB/s
Linux version 6.6.108 : 2.3 GB/s
Linux version 6.17.2 : 2.5 GB/s

These numbers were taken on a AMD Ryzen Embedded V1780B with 16 GB RAM.
We also noticed the same issue on other AMD and Intel machines.

openwrt[.]org offers kernel images on their website, I have also booted several openwrt kernel versions and noticed the same degradation.
I have also confirmed that the same issue occurs when running different versions of Ubuntu on the same machines.
I do not have the means to test this on ARM/other architectures.

We tried modifying to kernel config to disable CPU mitigations, setting the CPU frequency governor to performance, disabling kernel debugging and many such options etc. after which we noticed a small 3-5% improvement but we can no longer come near the older 4.x performance numbers.

Has anyone else observed similar performance regressions with tmpfs in recent kernel versions ?
Any pointers, explanations, or patches that could help identify or resolve this issue would be greatly appreciated.
I am happy to provide further details, logs, and build/test scripts if needed.

Best Regards,
Elison Niven

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