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Date: Mon, 22 Feb 2016 19:43:25 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To: Minchan Kim <minchan@...nel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
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 (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;
}
-ss
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