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Subject[PATCH 12/27] nfs: lower writeback threshold proportionally to dirty threshold
nfs_congestion_kb is to control the max allowed writeback and in-commit
pages. It's not reasonable for them to outnumber dirty and to-commit
pages. So each of them should not take more than 1/4 dirty threshold.

Considering that nfs_init_writepagecache() is called on fresh boot,
at the time dirty_thresh is much higher than the real dirty limit after
lots of user space memory consumptions, use 1/8 instead.

We might update nfs_congestion_kb when global dirty limit is changed
at runtime, but whatever, do it simple first.

CC: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
fs/nfs/write.c | 13 +++++++++++++
mm/page-writeback.c | 1 +
2 files changed, 14 insertions(+)
--- linux-next.orig/fs/nfs/write.c 2011-03-03 14:04:01.000000000 +0800
+++ linux-next/fs/nfs/write.c 2011-03-03 14:04:01.000000000 +0800
@@ -1651,6 +1651,9 @@ out:

int __init nfs_init_writepagecache(void)
{
+ unsigned long background_thresh;
+ unsigned long dirty_thresh;
+
nfs_wdata_cachep = kmem_cache_create("nfs_write_data",
sizeof(struct nfs_write_data),
0, SLAB_HWCACHE_ALIGN,
@@ -1688,6 +1691,16 @@ int __init nfs_init_writepagecache(void)
if (nfs_congestion_kb > 256*1024)
nfs_congestion_kb = 256*1024;

+ /*
+ * Limit to 1/8 dirty threshold, so that writeback+in_commit pages
+ * won't overnumber dirty+to_commit pages.
+ */
+ global_dirty_limits(&background_thresh, &dirty_thresh);
+ dirty_thresh <<= PAGE_SHIFT - 10;
+
+ if (nfs_congestion_kb > dirty_thresh / 8)
+ nfs_congestion_kb = dirty_thresh / 8;
+
return 0;
}

--- linux-next.orig/mm/page-writeback.c 2011-03-03 14:04:01.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-03-03 14:04:01.000000000 +0800
@@ -431,6 +431,7 @@ void global_dirty_limits(unsigned long *
*pbackground = background;
*pdirty = dirty;
}
+EXPORT_SYMBOL_GPL(global_dirty_limits);

/**
* bdi_dirty_limit - @bdi's share of dirty throttling threshold



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