[Xen-devel] A good way to speed up the xl destroy time(guest page scrubbing)

[Bob Liu <bob.liu@xxxxxxxxxx>]

Hey folks,

In recent months I've been working on speed up the 'xl des' time of XEN
guest with large RAM, but there is still no good solution yet.

I'm looking forward to get more suggestions and appreciate for all of
your input.

(1) The problem
When 'xl destory' a guest with large memory, we have to wait a long
time(~10 minutes for a guest with 1TB memory). Most of the time was
spent on page scrubbing, every page need to get scrubbed before free to
the heap_list(the buddy system).

(2) The way I've tired
1. When free a page to the buddy system only mark it with an new flag
'need_scrub' instead of scrubbing, so 'xl des' can return quickly.

2. Use all idle cpus to do the real page scrubbing in parallel. In:
static void idle_loop(void)
{
        iterate the &heap_list and scrub any 'need_scrub' page.
}

3. Also in the alloc_heap_page() path, 'need_scrub' pages can be
allocated and scrubbed.(If 'need_scrub' pages are skipped, 'xl create'
new guest may fail when the system is busy since no idle cpus can finish
the scrubbing.)

4. Problem of this way: Lock contention
The &heap_list is protected by heap_lock which is a spinlock.
alloc/free path may modify the heap list any time with heap_lock hold.

The idle_loop() need to iterate the heap list for every page scrubbing
(won't modify the list but will scrub page content), there is heavy lock
contention and slow down the alloc/free path.

5. Potential workaround
5.1 Use per-cpu list in idle_loop()
Delist a batch of pages from heap_list to a per-cpu list, then scrub the
per-cpu list and free back to heap_list.

But Jan disagree with this solution:
"You should really drop the idea of removing pages temporarily.
All you need to do is make sure a page being allocated and getting
simultaneously scrubbed by another CPU won't get passed to the
caller until the scrubbing finished."

Another reason was it's hard to say how many pages should be delisted to
per-cpu list.

5.2 Use more page flags
Konrad suggested to use more page flags and consider the 'cmpxchg'
instruction instead of spinlock for idle_loop() to iterate the &heap_list.
But 'cmpxchg' is only suitable to protect the content of every single
page, it's difficult to protect kinds of race conditions against a list.

(3) Other solutions for speed up page scrubbing
1. George suggested:
* Have a "clean" freelist and a "dirty" freelist
* When destroying a domain, simply move pages to the dirty freelist
* Have idle vcpus scrub the dirty freelist before going to sleep
 - ...

* In alloc_domheap_pages():
 - If there are pages on the "clean" freelist, allocate them
 - If there are no pages on the "clean" freelist but there are on the
"dirty" freelist, scrub pages from the "dirty" freelist synchronously.

But the lock contention is still a problem and may worse with two lists.

2. Delay page scrubbing to the page fault path
Which means a 'need_scrub' page won't be scrubbed until setting up the
page table mapping in page fault path. This is a populate way under linux.
But konrad mentioned this way was not suitable for Windows guest,
because Windows will access every page during boot up, the boot time of
windows might be slowed down.

Welcome any better ideas and thanks again for your patient to read this
long email.

-- 
Regards,
-Bob

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