Poor hard disk performance under kernel 4.15 series (HWE stack)

[indigocat]

Hi!

I've noticed that the disk I/O performance has dropped in the 4.15 series (Ubuntu 16.04 HWE), at least on mechanical hard drives; I'm working on an early Thinkpad T410s with 4Gb of RAM and a 1.8" 250Gb microSATA Toshiba hard drive.

Both 4.4 (lts-xenial) and 4.18 (liquorix) series are better performing than the 4.15 series, which shows a lot of r/w activity during application startup in comparison to the aforementioned kernels.

I've run some primitive benchmarks on all 3 kernel series, 3 passes each, showing the following average rates:

# Kernel 4.15.0-34
## I/O Scheduler: [deadline]
### $ time sh -c "dd if=/dev/zero of=testfile bs=1024k count=1k && sync && rm -f testfile"
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 29,1757 s, 36,8 MB/s
sh -c 0,01s user 1,33s system 4% cpu 31,724 total

# Kernel 4.18-liquorix (https://liquorix.net)
## I/O Scheduler: [deadline]
### time sh -c "dd if=/dev/zero of=testfile bs=1024k count=1k && sync && rm -f testfile"
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 23,7735 s, 45,2 MB/s
sh -c 0,15s user 1,25s system 4% cpu 28,193 total

# Kernel 4.4 lts-xenial
## I/O Scheduler: [deadline]
### $ time sh -c "dd if=/dev/zero of=testfile bs=1024k count=1k && sync && rm -f testfile"
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 23,9265 s, 44,9 MB/s
sh -c 0,00s user 1,68s system 5% cpu 32,312 total

As you can see, disk I/O is quite similar on both 4.4 and 4.18 series, but drops over a 25% on kernel 4.15.

It may not be noticeable on a modern machine with SSD drives, but it's quite a drag on mechanical drives. Even opening Emacs+Spacemacs takes over a minute under kernel 4.15, in contrast to the 20+ seconds under kernels 4.4 and 4.18-liquorix.

What further information would you need from me to pinpoint probable bottlenecks?

ProblemType: Bug
DistroRelease: elementary 0.4.1
Package: linux-image-4.15.0-34-generic 4.15.0-34.37~16.04.1 [origin: Ubuntu]
Uname: Linux 4.18.0-8.1-liquorix-amd64 x86_64
NonfreeKernelModules: nvidia_uvm nvidia wl
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CurrentDesktop: Pantheon
Date: Mon Sep 17 16:43:06 2018
InstallationDate: Installed on 2017-09-24 (358 days ago)
InstallationMedia: elementary OS 0.4.1 "Loki" - Stable amd64 (20170814)
SourcePackage: linux-signed-hwe
UpgradeStatus: No upgrade log present (probably fresh install)

[indigocat (indigocat)]

I did some more tests today.
Note: this is a mechanical MicroSATA laptop drive.

# WRITE
## 4.15.0-36

$ sync; dd if=/dev/zero of=tempfile bs=1M count=1024; sync
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 34,2693 s, 31,3 MB/s

$ sync; dd if=/dev/zero of=/tmp/tempfile bs=1M count=1024; sync
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 21,7465 s, 49,4 MB/s

$ sync; dd if=/dev/zero of=/tmp/tempfile bs=1M count=1024; sync
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 22,4577 s, 47,8 MB/s

Average: 42,83 MB/s WRITE

## 4.4.0-137

$ sync; dd if=/dev/zero of=/tmp/tempfile bs=1M count=1024; sync
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 19,0247 s, 56,4 MB/s

$ sync; dd if=/dev/zero of=/tmp/tempfile bs=1M count=1024; sync
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 20,5483 s, 52,3 MB/s

$ sync; dd if=/dev/zero of=/tmp/tempfile bs=1M count=1024; sync
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 20,5732 s, 52,2 MB/s

Average: 53,63 MB/s WRITE

# READ
## 4.15.0-36

$ sudo sysctl -w vm.drop_caches=3; dd if=/tmp/tempfile of=/dev/null bs=1M count=1024
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 24,635 s, 43,6 MB/s

$ sudo sysctl -w vm.drop_caches=3; dd if=/tmp/tempfile of=/dev/null bs=1M count=1024
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 24,4772 s, 43,9 MB/s

$ sudo sysctl -w vm.drop_caches=3; dd if=/tmp/tempfile of=/dev/null bs=1M count=1024
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 25,8508 s, 41,5 MB/s

Average: 43 MB/s READ

## 4.4.0-137

$ sudo sysctl -w vm.drop_caches=3; dd if=/tmp/tempfile of=/dev/null bs=1M count=1024
vm.drop_caches = 3
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 25,4324 s, 42,2 MB/s

$ sudo sysctl -w vm.drop_caches=3; dd if=/tmp/tempfile of=/dev/null bs=1M count=1024
vm.drop_caches = 3
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 25,1271 s, 42,7 MB/s

$ sudo sysctl -w vm.drop_caches=3; dd if=/tmp/tempfile of=/dev/null bs=1M count=1024
vm.drop_caches = 3
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 25,0382 s, 42,9 MB/s

Average: 42,6 MB/s READ

We could say that kernel 4.4 and 4.15 read speeds are almost the same, but write speed on kernel 4.15 is about 25% slower; this matters on older machines that use swap, perhaps not so much on higher-end machines relying on SSDs.

[indigocat (indigocat)]

Hello,

has anyone experienced this slowdown on similar hardware?