Re: [PATCH] block: BFQ default for single queue devices

[Bart Van Assche]

On Thu, 2018-10-04 at 22:39 +-0200, Paolo Valente wrote:
+AD4 No, kernel build is, for evident reasons, one of the workloads I cared
+AD4 most about.  Actually, I tried to focus on all my main
+AD4 kernel-development tasks, such as also git checkout, git merge, git
+AD4 grep, ...
+AD4 
+AD4 According to my test results, with BFQ these tasks are at least as
+AD4 fast as, or, in most system configurations, much faster than with the
+AD4 other schedulers.  Of course, at the same time the system also remains
+AD4 responsive with BFQ.
+AD4 
+AD4 You can repeat these tests using one of my first scripts in the S
+AD4 suite: kern+AF8-dev+AF8-tasks+AF8-vs+AF8-rw.sh (usually, the older the tests, the more
+AD4 hypertrophied the names I gave :) ).
+AD4 
+AD4 I stopped sharing also my kernel-build results years ago, because I
+AD4 went on obtaining the same, identical good results for years, and I'm
+AD4 aware that I tend to show and say too much stuff.

On my test setup building the kernel is slightly slower when using the BFQ
scheduler compared to using scheduler +ACI-none+ACI (kernel 4.18.12, NVMe SSD,
single CPU with 6 cores, hyperthreading disabled). I am aware that the
proposal at the start of this thread was to make BFQ the default for devices
with a single hardware queue and not for devices like NVMe SSDs that support
multiple hardware queues.

What I think is missing is measurement results for BFQ on a system with
multiple CPU sockets and against a fast storage medium. Eliminating
the host lock from the SCSI core yielded a significant performance
improvement for such storage devices. Since the BFQ scheduler locks and
unlocks bfqd-+AD4-lock for every dispatch operation it is very likely that BFQ
will slow down I/O for fast storage devices, even if their driver only
creates a single hardware queue.

Bart.