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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