Messages in this thread | | | Date | Wed, 14 Mar 2007 21:42:46 +0900 | From | Tomoki Sekiyama <> | Subject | [PATCH 0/3] VM throttling: avoid blocking occasional writers |
| |
Hi,
I ported the patch sent before to 2.6.21-rc3-mm2, so I'm resending it. ( Previous patch is available at http://marc.info/?l=linux-kernel&m=117223267512340&w=2 )
-Summary: I have observed a problem that write(2) can be blocked for a long time if a system has several disks and is under heavy I/O pressure. This patchset is to avoid the problem by introducing high/low water-mark algorithm to balance_dirty_pages() function.
-Example of the probrem:
There are two processes on a system which has two disks. Process-A writes heavily to disk-a, and process-B writes small data (e.g. log files) to disk-b occasionally. A portion of system memory, which is depends on vm.dirty_ratio (typically 40%), is filled up with Dirty and Writeback pages of disk-a.
In this situation, write(2) of process-B could be blocked for a very long time (more then 60 seconds), although the load of disk-b is quite low. In particular, the system would become quite slow, if disk-a is slow (e.g. backup to an USB disk).
This seems to be the same problem as discussed in LKML: http://marc.theaimsgroup.com/?t=115559902900003 and http://marc.theaimsgroup.com/?t=117182340400003
-Cause:
I found this problem is caused by the balance_dirty_pages().
While Dirty+Writeback pages get more than 40% of memory, process-B is blocked in balance_dirty_pages() until writeback of some (`write_chunk', typically = 1536) dirty pages on disk-b is started.
However, because disk-b has only a few dirty pages, the process-B will be blocked until writeback to disk-a is completed and Dirty+Writeback goes below 40%.
-Solution:
I consider that all of the dirty pages for the disk have been written back and that the disk is clean if a process cannot write 'write_chunk' pages in balance_dirty_pages().
To avoid using up the free memory with dirty pages by passing blocking, this patchset adds a new threshold named vm.dirty_limit_ratio to sysctl.
It modifies balance_dirty_pages() not to block when the amount of Dirty+Writeback is less than vm.dirty_limit_ratio percent of the memory. In the other cases, writers are throttled as current Linux does.
In this patchset, vm.dirty_limit_ratio, instead of vm.dirty_ratio, is used as the clamping level of Dirty+Writeback. And, vm.dirty_ratio is used as the level at which a writers will itself start writeback of the dirty pages.
-Testing Results:
In the situation explained in "Example of the problem" section, I measured time of write(2)ing to disk-b. The write was completed by 30ms or less under the kernel with this patchset.
When nr_requests is set too high (e.g. 8192), Dirty+Writeback grows near vm.dirty_limit_ratio(45% of system memory by defaults). In that case, write(2) sometimes took about 1 second.
This patchset can be applied to 2.6.21-rc3-mm2. It consists of 3 pieces:
1/3 - add a sysctl variable `vm.dirty_limit_ratio' 2/3 - modify get_dirty_limits() to return the limit of dirty pages. 3/3 - break out of balance_dirty_pages() loop if the disk doesn't have remaining dirty pages, if Dirty+Writeback < vm.dirty_limit_ratio.
-- Tomoki Sekiyama Hitachi, Ltd., Systems Development Laboratory
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |