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We currently preallocate scatterlists which have 2048 entries for each command. For a small device with just 1 queue this results in: 8 MB = 32 bytes per sg * 2048 entries * 128 cmd When mq is turned on and we increase the virtqueue_size so we can handle commands from multiple queues in parallel, then this can sky rocket. This patch allows us to dynamically allocate the scatterlist like is done with drivers like NVMe and SCSI. For small IO (4-16K) IOPs testing, we didn't see any regressions, but for throughput testing we sometimes saw a 2-5% regression when the backend device was very fast (8 NVMe drives in a MD RAID0 config or a memory backed device). As a result this patch makes the dynamic allocation feature a modparam so userspace can decide how it wants to balance mem use and perf. Signed-off-by: Mike Christie <michael.christie@oracle.com> Message-Id: <20241203191705.19431-6-michael.christie@oracle.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.