Re: [2.4] heavy-load under swap space shortage

Marcelo Tosatti
2004-03-14T19:49:15Z

Hi kernel colleagues,

At first I was skeptic to the inclusion of this patch in v2.4 (due to the
freeze), but after thinking a bit more about I have a few points in favour
of this modification (read Nomura's message below and the patch to know
what I'm talking about):

- It is off by default
- It is very simple (non intrusive), it just changes the point in which
anonymous pages are inserted into the LRU.
- When turned on, I dont see it being a reason for introducing new
bugs.

What you think of this?

On Wed, 10 Mar 2004 j-nomura@ce.jp.nec.com wrote:

> After discussion with Hugh and recommendation from Andrea,
> it turns out that Andrea's 05_vm_22_vm-anon-lru-3 in 2.4.23aa2 solves
> the problem.
> ftp://ftp.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.23aa2/05_vm_22_vm-anon-lru-3
>
> The patch adds a sysctl which accelerate the performance on
> huge memory machine. It doesn't affect anything if turned off.
>
> Marcelo, could you apply this to 2.4.26-pre?
> (I attached the slightly modified patch in which the feature is turned
> off by default and which is cleanly applied to bk tree.)
>
>
> My test case was:
> - there is a process with large anonymous mapping
> - there are large amount of page caches and active I/O processes
> - there are not much of file mappings
>
> So the problem happens in this way:
> - shrink_cache tries scanning inactive list in which most of pages
> are anonymous mapped
> - it soon fall into swap_out because of too many anonymous pages
> - when no free swap space, it hardly frees anything
> - it retries again but soon calls swap_out again and again
>
> Without the patch, snapshot of readprofile looks like:
> 3590781 total
> 3289271 swap_out
> 212029 smp_call_function
> 22598 shrink_cache
> 21833 lru_cache_add
> 7787 get_user_pages
>
> Most of the time was spent in swap_out. (contention on pagetable_lock)
>
> After applying the patch, the snapshot is like:
> 17420 total
> 3929 copy_page
> 3677 statm_pgd_range
> 1317 try_to_free_buffers
> 1312 __copy_user
> 593 scsi_make_request
>
> Best regards.

--- linux/include/linux/swap.h 2004/02/19 04:12:39 1.1.1.26
+++ linux/include/linux/swap.h 2004/03/10 10:09:11
@@ -116,7 +116,7 @@ extern void swap_setup(void);
extern wait_queue_head_t kswapd_wait;
extern int FASTCALL(try_to_free_pages_zone(zone_t *, unsigned int));
extern int FASTCALL(try_to_free_pages(unsigned int));
-extern int vm_vfs_scan_ratio, vm_cache_scan_ratio, vm_lru_balance_ratio, vm_passes, vm_gfp_debug, vm_mapped_ratio;
+extern int vm_vfs_scan_ratio, vm_cache_scan_ratio, vm_lru_balance_ratio, vm_passes, vm_gfp_debug, vm_mapped_ratio, vm_anon_lru;

/* linux/mm/page_io.c */
extern void rw_swap_page(int, struct page *);
--- linux/include/linux/sysctl.h 2004/02/19 04:12:39 1.1.1.23
+++ linux/include/linux/sysctl.h 2004/03/10 10:09:11
@@ -156,6 +156,7 @@ enum
VM_MAPPED_RATIO=20, /* amount of unfreeable pages that triggers swapout */
VM_LAPTOP_MODE=21, /* kernel in laptop flush mode */
VM_BLOCK_DUMP=22, /* dump fs activity to log */
+ VM_ANON_LRU=23, /* immediatly insert anon pages in the vm page lru */
};

--- linux/kernel/sysctl.c 2003/12/02 04:48:47 1.1.1.22
+++ linux/kernel/sysctl.c 2004/03/10 10:09:12
@@ -287,6 +287,8 @@ static ctl_table vm_table[] = {
&vm_cache_scan_ratio, sizeof(int), 0644, NULL, &proc_dointvec},
{VM_MAPPED_RATIO, "vm_mapped_ratio",
&vm_mapped_ratio, sizeof(int), 0644, NULL, &proc_dointvec},
+ {VM_ANON_LRU, "vm_anon_lru",
+ &vm_anon_lru, sizeof(int), 0644, NULL, &proc_dointvec},
{VM_LRU_BALANCE_RATIO, "vm_lru_balance_ratio",
&vm_lru_balance_ratio, sizeof(int), 0644, NULL, &proc_dointvec},
{VM_PASSES, "vm_passes",
--- linux/mm/memory.c 2003/12/02 04:48:47 1.1.1.31
+++ linux/mm/memory.c 2004/03/10 10:09:12
@@ -984,7 +984,8 @@ static int do_wp_page(struct mm_struct *
if (PageReserved(old_page))
++mm->rss;
break_cow(vma, new_page, address, page_table);
- lru_cache_add(new_page);
+ if (vm_anon_lru)
+ lru_cache_add(new_page);

/* Free the old page.. */
new_page = old_page;
@@ -1215,7 +1216,8 @@ static int do_anonymous_page(struct mm_s
mm->rss++;
flush_page_to_ram(page);
entry = pte_mkwrite(pte_mkdirty(mk_pte(page, vma->vm_page_prot)));
- lru_cache_add(page);
+ if (vm_anon_lru)
+ lru_cache_add(page);
mark_page_accessed(page);
}

@@ -1270,7 +1272,8 @@ static int do_no_page(struct mm_struct *
}
copy_user_highpage(page, new_page, address);
page_cache_release(new_page);
- lru_cache_add(page);
+ if (vm_anon_lru)
+ lru_cache_add(page);
new_page = page;
}

--- linux/mm/vmscan.c 2004/02/19 04:12:33 1.1.1.32
+++ linux/mm/vmscan.c 2004/03/10 10:09:13
@@ -65,6 +65,27 @@ int vm_lru_balance_ratio = 2;
int vm_vfs_scan_ratio = 6;

/*
+ * "vm_anon_lru" select if to immdiatly insert anon pages in the
+ * lru. Immediatly means as soon as they're allocated during the
+ * page faults.
+ *
+ * If this is set to 0, they're inserted only after the first
+ * swapout.
+ *
+ * Having anon pages immediatly inserted in the lru allows the
+ * VM to know better when it's worthwhile to start swapping
+ * anonymous ram, it will start to swap earlier and it should
+ * swap smoother and faster, but it will decrease scalability
+ * on the >16-ways of an order of magnitude. Big SMP/NUMA
+ * definitely can't take an hit on a global spinlock at
+ * every anon page allocation. So this is off by default.
+ *
+ * Low ram machines that swaps all the time want to turn
+ * this on (i.e. set to 1).
+ */
+int vm_anon_lru = 1;
+
+/*
* The swap-out function returns 1 if it successfully
* scanned all the pages it was asked to (`count').
* It returns zero if it couldn't do anything,