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SubjectRe: [PATCH][BUGFIX][RFC] fix soft lock up at NFS mount by making limitation of dentry_unused
David Chinner wrote:
> On Thu, Mar 06, 2008 at 01:41:29PM +0900, Kentaro Makita wrote:
....
>> 100,000,000 is possible number on large systems.
>>
>> This problem already happend on our system.
>> Therefore, we need a limitation of dentry_unused.
>
> No, we need a smarter free list structure. There have been several attempts
> at this in the past. Two that I can recall off the top of my head:
>
> - per node unused LRUs
> - per superblock unusued LRUs

I know there is such attempt already, but they are not in main-line.
I think this is not a smart but simple way to avoid this ploblem.
>
> I guess we need to revisit this again, because limiting the size of
> the cache like this is not an option.
>
>> I feel we need more tests to determine resonable value to any system.
>> So, please test.
> .....
>> Tested on Intel Itanium 2 9050 (dualcore) x12 MEM 24GB , kernel-2.6.25-rc4
>> I found no peformance regression in my tests.
>
> Try something that relies on leaving the working set on the unused
> list, like NFS server benchmarks that have a working set of tens of
> million of files....
>
Okay, I'll try some benchmarks and report results...
>> Signed-off-by: Kentaro Makita <k-makita@np.css.fujitsu.com>
>> ---
>> fs/dcache.c | 7 +++++++
>> 1 files changed, 7 insertions(+)
>> diff -rupN -X linux-2.6.25-rc4/Documentation/dontdiff linux-2.6.25-rc4/fs/dcache.c linux-2.6.25-rc4mod/fs/dcache.c
>> --- linux-2.6.25-rc4/fs/dcache.c 2008-03-05 13:33:54.000000000 +0900
>> +++ linux-2.6.25-rc4mod/fs/dcache.c 2008-03-05 16:47:18.000000000 +0900
......
>> @@ -214,6 +217,10 @@ repeat:
>> }
>> spin_unlock(&dentry->d_lock);
>> spin_unlock(&dcache_lock);
>> + /* Prune unused dentry over threshold level */
>> + int nr_in_use = (dentry_stat.nr_dentry - dentry_stat.nr_unused);
>> + if (dentry_stat.nr_dentry > nr_in_use * dentry_unused_ratio / 100)
>> + prune_dcache(dentry_stat.nr_unused * 5 / 100 , NULL);
>
> nr_in_use is going to overflow 32 bits with this calculation.
Oh, I simply mistake. I fix it at this post.
>
> Cheers,
>
> Dave.
Best Regards,
Kentaro Makita

Signed-off-by: Kentaro Makita <k-makita@np.css.fujitsu.com>
---
dcache.c | 7 +++++++
1 files changed, 7 insertions(+)
diff -rupN -X linux-2.6.25-rc4/Documentation/dontdiff linux-2.6.25-rc4/fs/dcache.c linux-2.6.25-rc4mod/fs/dcache.c
--- linux-2.6.25-rc4/fs/dcache.c 2008-03-05 13:33:54.000000000 +0900
+++ linux-2.6.25-rc4mod/fs/dcache.c 2008-03-06 15:27:22.000000000 +0900
@@ -42,6 +42,8 @@ __cacheline_aligned_in_smp DEFINE_SEQLOC

EXPORT_SYMBOL(dcache_lock);

+/* threshold to limit dentry_unused */
+unsigned int dentry_unused_ratio = 10000;
static struct kmem_cache *dentry_cache __read_mostly;

#define DNAME_INLINE_LEN (sizeof(struct dentry)-offsetof(struct dentry,d_iname))
@@ -61,6 +63,7 @@ static unsigned int d_hash_mask __read_m
static unsigned int d_hash_shift __read_mostly;
static struct hlist_head *dentry_hashtable __read_mostly;
static LIST_HEAD(dentry_unused);
+static void prune_dcache(int count, struct super_block *sb);

/* Statistics gathering. */
struct dentry_stat_t dentry_stat = {
@@ -214,6 +217,10 @@ repeat:
}
spin_unlock(&dentry->d_lock);
spin_unlock(&dcache_lock);
+ /* Prune unused dentry over threshold level */
+ int nr_in_use = (dentry_stat.nr_dentry - dentry_stat.nr_unused);
+ if (dentry_stat.nr_dentry > nr_in_use * (dentry_unused_ratio / 100))
+ prune_dcache(dentry_stat.nr_unused * 5 / 100 , NULL);
return;

unhash_it:


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