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This patch is to reduce the performance drop depth observed on SATA HDD when ATA PT command is outstanding. Driver returns IO commands with status "SAM_STAT_BUSY" whenever ATA PT command is outstanding. With this, IO commands will be retried until this outstanding ATA PT to complete and hence we will observe drop in performance. As the driver is completing the subsequent IOs commands with SAM_STAT_BUSY status, these IOs has to go though the block layer. Hence it adds latency to the IOs and large performance drop is observed. So to reduce this performance dropp, added improvement in driver to return the subsequent IOs with SCSI_MLQUEUE_DEVICE_BUSY status instead of completing the IOs with SAM_STAT_BUSY status when ATA PT command is outstanding. Sending command back with SCSI_MLQUEUE_DEVICE_BUSY does not go through complete block layer stack (as scsi_done won't be called) SML will immediately retry the command and this method will avoid latency of block layer stack and the performance impact will be reduced. On Local setup, ran 512k sequential read IO operation on HGST SATA drive with existing driver & with this improvement drivers and here is the result, 1. With existing driver: IOs are running at bandwidth of ~230 rMB/s and whenever any ATA PT command is outstanding (e.g issued from systemd-udevd daemon) then this bandwidth drops to ~150 rMB/s. 2. With this improvement driver: IOs are running at bandwidth of ~230 rMB/s and whenever any ATA PT command is outstanding then this bandwidth drops to just ~190 rMB/s. Signed-off-by: Suganath Prabu <suganath-prabu.subramani@broadcom.com> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com> |
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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 Restructured Text 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.