Re: process creation time increases linearly with shmem

[Nick Piggin <nickpiggin@yahoo.com.au>]

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Ray Fucillo wrote:
> Nick Piggin wrote:
> 
>> fork() can be changed so as not to set up page tables for
>> MAP_SHARED mappings. I think that has other tradeoffs like
>> initially causing several unavoidable faults reading
>> libraries and program text.
>>
>> What kind of application are you using?
> 
> 
> The application is a database system called Caché.  We allocate a large 
> shared memory segment for database cache, which in a large production 
> environment may realistically be 1+GB on 32-bit platforms and much 
> larger on 64-bit.  At these sizes fork() is taking hundreds of 
> miliseconds, which can become a noticeable bottleneck for us.  This 
> performance characteristic seems to be unique to Linux vs other Unix 
> implementations.
> 
> 

As Andi said, hugepages might be a very nice feature for you guys
to look into and might potentially give a performance increase with
reduced TLB pressure, not only your immediate fork problem.

Anyway, the attached patch is something you could try testing. If
you do so, then I would be very interested to see performance results.

Thanks,
Nick

-- 
SUSE Labs, Novell Inc.


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Index: linux-2.6/kernel/fork.c
===================================================================
--- linux-2.6.orig/kernel/fork.c	2005-08-04 15:24:36.000000000 +1000
+++ linux-2.6/kernel/fork.c	2005-08-26 00:20:50.000000000 +1000
@@ -256,7 +256,6 @@ static inline int dup_mmap(struct mm_str
 		 * Note that, exceptionally, here the vma is inserted
 		 * without holding mm->mmap_sem.
 		 */
-		spin_lock(&mm->page_table_lock);
 		*pprev = tmp;
 		pprev = &tmp->vm_next;
 
@@ -265,8 +264,11 @@ static inline int dup_mmap(struct mm_str
 		rb_parent = &tmp->vm_rb;
 
 		mm->map_count++;
-		retval = copy_page_range(mm, current->mm, tmp);
-		spin_unlock(&mm->page_table_lock);
+		if (!(file && (tmp->vm_flags & VM_SHARED))) {
+			spin_lock(&mm->page_table_lock);
+			retval = copy_page_range(mm, current->mm, tmp);
+			spin_unlock(&mm->page_table_lock);
+		}
 
 		if (tmp->vm_ops && tmp->vm_ops->open)
 			tmp->vm_ops->open(tmp);