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SubjectRe: process creation time increases linearly with shmem
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.

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);
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