From: Mel Gorman <mgorman@suse.de>
To: Dan Streetman <ddstreet@ieee.org>
Cc: Hugh Dickins <hughd@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
Michal Hocko <mhocko@suse.cz>,
Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>,
Shaohua Li <shli@fusionio.com>, Weijie Yang <weijieut@gmail.com>,
Linux-MM <linux-mm@kvack.org>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] swap: change swap_info singly-linked list to list_head
Date: Fri, 25 Apr 2014 09:38:30 +0100 [thread overview]
Message-ID: <20140425083830.GY23991@suse.de> (raw)
In-Reply-To: <CALZtONCa3jLrYkPSFPNnV84zePxFtdkWJBu092ScgUe2AugMxQ@mail.gmail.com>
On Thu, Apr 24, 2014 at 02:48:43PM -0400, Dan Streetman wrote:
> >> <SNIP>
> >> - }
> >> -
> >> + list_for_each(tmp, &swap_list_head) {
> >> + si = list_entry(tmp, typeof(*si), list);
> >> spin_lock(&si->lock);
> >> - if (!si->highest_bit) {
> >> - spin_unlock(&si->lock);
> >> - continue;
> >> - }
> >> - if (!(si->flags & SWP_WRITEOK)) {
> >> + if (!si->highest_bit || !(si->flags & SWP_WRITEOK)) {
> >> spin_unlock(&si->lock);
> >> continue;
> >> }
> >>
> >> - swap_list.next = next;
> >> + /*
> >> + * rotate the current swap_info that we're going to use
> >> + * to after any other swap_info that have the same prio,
> >> + * so that all equal-priority swap_info get used equally
> >> + */
> >> + next = si;
> >> + list_for_each_entry_continue(next, &swap_list_head, list) {
> >> + if (si->prio != next->prio)
> >> + break;
> >> + list_rotate_left(&si->list);
> >> + next = si;
> >> + }
> >>
> >
> > The list manipulations will be a lot of cache writes as the list is shuffled
> > around. On slow storage I do not think this will be noticable but it may
> > be noticable on faster swap devices that are SSD based. I've added Shaohua
> > Li to the cc as he has been concerned with the performance of swap in the
> > past. Shaohua, can you run this patchset through any of your test cases
> > with the addition that multiple swap files are used to see if the cache
> > writes are noticable? You'll need multiple swap files, some of which are
> > at equal priority so the list shuffling logic is triggered.
>
> One performance improvement could be instead of rotating the current
> entry past each following same-prio entry, just scan to the end of the
> same-prio entries and move the current entry there; that would skip
> the extra writes. Especially since this code will run for each
> get_swap_page(), no need for any unnecessary writes.
>
Shaohua is the person that would be most sensitive to performance problems
in this area and his tests are in the clear. If he's happy then I don't
think there is justification for changing the patch as-is.
> >
> >> spin_unlock(&swap_lock);
> >> /* This is called for allocating swap entry for cache */
> >> offset = scan_swap_map(si, SWAP_HAS_CACHE);
> >> spin_unlock(&si->lock);
> >> if (offset)
> >> - return swp_entry(type, offset);
> >> + return swp_entry(si->type, offset);
> >> spin_lock(&swap_lock);
> >> - next = swap_list.next;
> >> + /*
> >> + * shouldn't really have got here, but for some reason the
> >> + * scan_swap_map came back empty for this swap_info.
> >> + * Since we dropped the swap_lock, there may now be
> >> + * non-full higher prio swap_infos; let's start over.
> >> + */
> >> + tmp = &swap_list_head;
> >> }
> >
> > Has this ever triggered? The number of swap pages was examined under the
> > swap lock so no other process should have been iterating through the
> > swap files. Once a candidate was found, the si lock was acquired for the
> > swap scan map so nothing else should have raced with it.
>
> Well scan_swap_map() does drop the si->lock if it has any trouble at
> all finding an offset to use, so I think it's possible that for a
> nearly-full si multiple concurrent get_swap_page() calls could enter
> scan_swap_map() with the same si, only some of them actually get pages
> from the si and then the si becomes full, and the other threads in
> scan_swap_map() see it's full and exit in failure. I can update the
> code comment there to better indicate why it was reached, instead of
> just saying "we shouldn't have got here" :-)
>
With the updates to some comments then feel free to add
Acked-by: Mel Gorman <mgorman@suse.de>
> It may also be worth trying to get a better indicator of "available"
> swap_info_structs for use in get_swap_page(), either by looking at
> something other than si->highest_bit and/or keeping the si out of the
> prio_list until it's actually available for use, not just has a single
> entry free. However, that probably won't be simple and might be
> better as a separate patch to the rest of these changes.
>
I agree that it is likely outside the scope of what this series is meant
to accomplish.
--
Mel Gorman
SUSE Labs
next prev parent reply other threads:[~2014-04-25 8:38 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-13 10:42 [PATCH] mm: swap: Use swapfiles in priority order Mel Gorman
2014-02-13 15:58 ` Weijie Yang
2014-02-14 10:17 ` Mel Gorman
2014-02-14 13:33 ` Weijie Yang
[not found] ` <loom.20140214T135753-812@post.gmane.org>
[not found] ` <CABdxLJHS5kw0rpD=+77iQtc6PMeRXoWnh-nh5VzjjfGHJ5wLGQ@mail.gmail.com>
2014-02-24 8:28 ` Hugh Dickins
2014-04-12 21:00 ` [PATCH 0/2] swap: simplify/fix swap_list handling and iteration Dan Streetman
2014-04-12 21:00 ` [PATCH 1/2] swap: change swap_info singly-linked list to list_head Dan Streetman
2014-04-23 10:34 ` Mel Gorman
2014-04-24 0:17 ` Shaohua Li
2014-04-24 8:30 ` Mel Gorman
2014-04-24 18:48 ` Dan Streetman
2014-04-25 4:15 ` Weijie Yang
2014-05-02 20:00 ` Dan Streetman
2014-05-04 9:39 ` Bob Liu
2014-05-04 20:16 ` Dan Streetman
2014-04-25 8:38 ` Mel Gorman [this message]
2014-04-12 21:00 ` [PATCH 2/2] swap: use separate priority list for available swap_infos Dan Streetman
2014-04-23 13:14 ` Mel Gorman
2014-04-24 17:52 ` Dan Streetman
2014-04-25 8:49 ` Mel Gorman
2014-05-02 19:02 ` [PATCHv2 0/4] swap: simplify/fix swap_list handling and iteration Dan Streetman
2014-05-02 19:02 ` [PATCHv2 1/4] swap: change swap_info singly-linked list to list_head Dan Streetman
2014-05-02 19:02 ` [PATCH 2/4] plist: add helper functions Dan Streetman
2014-05-12 10:35 ` Mel Gorman
2014-05-02 19:02 ` [PATCH 3/4] plist: add plist_rotate Dan Streetman
2014-05-06 2:18 ` Steven Rostedt
2014-05-06 20:12 ` Dan Streetman
2014-05-06 20:39 ` Steven Rostedt
2014-05-06 21:47 ` Dan Streetman
2014-05-06 22:43 ` Steven Rostedt
2014-05-02 19:02 ` [PATCH 4/4] swap: change swap_list_head to plist, add swap_avail_head Dan Streetman
2014-05-05 15:51 ` Dan Streetman
2014-05-05 19:13 ` Steven Rostedt
2014-05-05 19:38 ` Peter Zijlstra
2014-05-09 20:42 ` [PATCH] plist: make CONFIG_DEBUG_PI_LIST selectable Dan Streetman
2014-05-09 21:17 ` Steven Rostedt
2014-05-12 11:11 ` [PATCH 4/4] swap: change swap_list_head to plist, add swap_avail_head Mel Gorman
2014-05-12 13:00 ` Dan Streetman
2014-05-12 16:38 ` [PATCHv3 0/4] swap: simplify/fix swap_list handling and iteration Dan Streetman
2014-05-12 16:38 ` [PATCHv2 1/4] swap: change swap_info singly-linked list to list_head Dan Streetman
2014-05-12 16:38 ` [PATCH 2/4] plist: add helper functions Dan Streetman
2014-05-12 16:38 ` [PATCHv2 3/4] plist: add plist_requeue Dan Streetman
2014-05-13 10:33 ` Mel Gorman
2014-05-12 16:38 ` [PATCHv2 4/4] swap: change swap_list_head to plist, add swap_avail_head Dan Streetman
2014-05-13 10:34 ` Mel Gorman
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20140425083830.GY23991@suse.de \
--to=mgorman@suse.de \
--cc=akpm@linux-foundation.org \
--cc=ddstreet@ieee.org \
--cc=ehrhardt@linux.vnet.ibm.com \
--cc=hughd@google.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhocko@suse.cz \
--cc=shli@fusionio.com \
--cc=weijieut@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).