Each `blkif` has a free pages pool for the grant mapping. The size of the pool starts from zero and be increased on demand while processing the I/O requests. If current I/O requests handling is finished or 100 milliseconds has passed since last I/O requests handling, it checks and shrinks the pool to not exceed the size limit, `max_buffer_pages`. Therefore, `blkfront` running guests can cause a memory pressure in the `blkback` running guest by attaching a large number of block devices and inducing I/O. System administrators can avoid such problematic situations by limiting the maximum number of devices each guest can attach. However, finding the optimal limit is not so easy. Improper set of the limit can result in the memory pressure or a resource underutilization. This commit avoids such problematic situations by shrinking the pools aggressively (further the limit) for a while (users can set this duration via a module parameter) if a memory pressure is detected. Base Version ------------ This patch is based on v5.4. A complete tree is also available at my public git repo: https://github.com/sjp38/linux/tree/blkback_aggressive_shrinking_v2 Patch History ------------- Changes from v1 (https://lore.kernel.org/xen-devel/20191204113419.2298-1-sjpark@xxxxxxxxxx/) - Adjust the description to not use the term, `arbitrarily` (suggested by Paul Durrant) - Specify time unit of the duration in the parameter description, (suggested by Maximilian Heyne) - Change default aggressive shrinking duration from 1ms to 10ms - Merged two patches into one single patch SeongJae Park (1): xen/blkback: Aggressively shrink page pools if a memory pressure is detected drivers/block/xen-blkback/blkback.c | 35 +++++++++++++++++++++++++++-- 1 file changed, 33 insertions(+), 2 deletions(-) -- 2.17.1
