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In below scenario blkio cgroup does not work as per their assigned weights :- 1. When the underlying device is nonrotational with a single HW queue with depth of >= CFQ_HW_QUEUE_MIN 2. When the use case is forming two blkio cgroups cg1(weight 1000) & cg2(wight 100) and two processes(file1 and file2) doing sync IO in their respective blkio cgroups. For above usecase result of fio (without this patch):- file1: (groupid=0, jobs=1): err= 0: pid=685: Thu Jan 1 19:41:49 1970 write: IOPS=1315, BW=41.1MiB/s (43.1MB/s)(1024MiB/24906msec) <...> file2: (groupid=0, jobs=1): err= 0: pid=686: Thu Jan 1 19:41:49 1970 write: IOPS=1295, BW=40.5MiB/s (42.5MB/s)(1024MiB/25293msec) <...> // both the process BW is equal even though they belong to diff. cgroups with weight of 1000(cg1) and 100(cg2) In above case (for non rotational NCQ devices), as soon as the request from cg1 is completed and even though it is provided with higher set_slice=10, because of CFQ algorithm when the driver tries to fetch the request, CFQ expires this group without providing any idle time nor weight priority and schedules another cfq group (in this case cg2). And thus both cfq groups(cg1 & cg2) keep alternating to get the disk time and hence loses the cgroup weight based scheduling. Below patch gives a chance to cfq algorithm (cfq_arm_slice_timer) to arm the slice timer in case group_idle is enabled. In case if group_idle is also not required (including for nonrotational NCQ drives), we need to explicitly set group_idle = 0 from sysfs for such cases. With this patch result of fio(for above usecase) :- file1: (groupid=0, jobs=1): err= 0: pid=690: Thu Jan 1 00:06:08 1970 write: IOPS=1706, BW=53.3MiB/s (55.9MB/s)(1024MiB/19197msec) <..> file2: (groupid=0, jobs=1): err= 0: pid=691: Thu Jan 1 00:06:08 1970 write: IOPS=1043, BW=32.6MiB/s (34.2MB/s)(1024MiB/31401msec) <..> // In this processes BW is as per their respective cgroups weight. Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org> Signed-off-by: Jens Axboe <axboe@kernel.dk> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.