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Date: Tue, 23 Feb 2016 17:25:32 +0900
From: Minchan Kim <minchan@...nel.org>
To: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
CC: Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Joonsoo Kim <js1304@...il.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH v2 3/3] mm/zsmalloc: increase ZS_MAX_PAGES_PER_ZSPAGE
On Mon, Feb 22, 2016 at 07:43:25PM +0900, Sergey Senozhatsky wrote:
> On (02/22/16 13:41), Minchan Kim wrote:
> [..]
> > > oh, sure.
> > >
> > > so let's keep dynamic page allocation out of sight for now.
> > > I'll do more tests with the increase ORDER and if it's OK then
> > > hopefully we can just merge it, it's quite simple and shouldn't
> > > interfere with any of the changes you are about to introduce.
> >
> > Thanks.
> >
> > And as another idea, we could try fallback approach that
> > we couldn't meet nr_pages to minimize wastage so let's fallback
> > to order-0 page like as-is. It will enhance, at least than now
> > with small-amount of code compared to dynmaic page allocation.
>
>
> speaking of fallback,
> with bigger ZS_MAX_ZSPAGE_ORDER 'normal' classes also become bigger.
>
> PATCHED
>
> 6 128 0 1 96 78 3 1
> 7 144 0 1 256 104 9 9
> 8 160 0 1 128 80 5 5
> 9 176 0 1 256 78 11 11
> 10 192 1 1 128 99 6 3
> 11 208 0 1 256 52 13 13
> 12 224 1 1 512 472 28 7
> 13 240 0 1 256 70 15 15
> 14 256 1 1 64 49 4 1
> 15 272 0 1 60 48 4 1
>
>
> BASE
>
> 6 128 0 1 96 83 3 1
> 7 144 0 1 170 113 6 3
> 8 160 0 1 102 72 4 2
> 9 176 1 0 93 75 4 4
> 10 192 0 1 128 104 6 3
> 11 208 1 1 78 52 4 2
> 12 224 1 1 511 475 28 4
> 13 240 1 1 85 73 5 1
> 14 256 1 1 64 53 4 1
> 15 272 1 0 45 43 3 1
>
>
> _techically_, zsmalloc is correct.
> for instance, in 11 pages we can store 4096 * 11 / 176 == 256 objects.
> 256 * 176 == 45056, which is 4096 * 11. so if zspage for class_size 176 will contain 11
> order-0 pages, we can count on 0 bytes of unused space once zspage will become ZS_FULL.
>
> but it's ugly, because I think this will introduce bigger internal fragmentation, which,
> in some cases, can be handled by compaction, but I'd prefer to touch only ->huge classes
> and keep the existing behaviour for normal classes.
>
> so I'm currently thinking of doing something like this
>
> #define ZS_MAX_ZSPAGE_ORDER 2
> #define ZS_MAX_HUGE_ZSPAGE_ORDER 4
> #define ZS_MAX_PAGES_PER_ZSPAGE (_AC(1, UL) << ZS_MAX_ZSPAGE_ORDER)
> #define ZS_MAX_PAGES_PER_HUGE_ZSPAGE (_AC(1, UL) << ZS_MAX_HUGE_ZSPAGE_ORDER)
>
>
> so, normal classes have ORDER of 2. huge classes, however, as a fallback, can grow
> up to ZS_MAX_HUGE_ZSPAGE_ORDER pages.
>
>
> extend only ->huge classes: pages == 1 && get_maxobj_per_zspage(class_size, pages) == 1.
>
> like this:
>
> static int __get_pages_per_zspage(int class_size, int max_pages)
> {
> int i, max_usedpc = 0;
> /* zspage order which gives maximum used size per KB */
> int max_usedpc_order = 1;
>
> for (i = 1; i <= max_pages; i++) {
> int zspage_size;
> int waste, usedpc;
>
> zspage_size = i * PAGE_SIZE;
> waste = zspage_size % class_size;
> usedpc = (zspage_size - waste) * 100 / zspage_size;
>
> if (usedpc > max_usedpc) {
> max_usedpc = usedpc;
> max_usedpc_order = i;
> }
> }
>
> return max_usedpc_order;
> }
>
> static int get_pages_per_zspage(int class_size)
> {
> /* normal class first */
> int pages = __get_pages_per_zspage(class_size,
> ZS_MAX_PAGES_PER_ZSPAGE);
>
> /* test if the class is ->huge and try to turn it into a normal one */
> if (pages == 1 &&
> get_maxobj_per_zspage(class_size, pages) == 1) {
> pages = __get_pages_per_zspage(class_size,
> ZS_MAX_PAGES_PER_HUGE_ZSPAGE);
> }
>
> return pages;
> }
>
That sounds like a plan but at a first glance, my worry is we might need
some special handling related to objs_per_zspage and pages_per_zspage
because currently, we have assumed all of zspages in a class has same
number of subpages so it might make it ugly.
Hmm, at least, I need to check code how it makes ugly.
If you think it's not trouble, please send a patch.
As well, please write down why order-4 for MAX_ZSPAGES is best
if you resend it as formal patch.
Thanks.
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