2015-03-17 17:22 GMT+09:00 Roman Peniaev <r.peniaev@xxxxxxxxx>:
> On Tue, Mar 17, 2015 at 4:29 PM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
>> On Tue, Mar 17, 2015 at 02:12:14PM +0900, Roman Peniaev wrote:
>>> On Tue, Mar 17, 2015 at 1:56 PM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
>>> > On Fri, Mar 13, 2015 at 09:12:55PM +0900, Roman Pen wrote:
>>> >> If suitable block can't be found, new block is allocated and put into a head
>>> >> of a free list, so on next iteration this new block will be found first.
>>> >>
>>> >> That's bad, because old blocks in a free list will not get a chance to be fully
>>> >> used, thus fragmentation will grow.
>>> >>
>>> >> Let's consider this simple example:
>>> >>
>>> >> #1 We have one block in a free list which is partially used, and where only
>>> >> one page is free:
>>> >>
>>> >> HEAD |xxxxxxxxx-| TAIL
>>> >> ^
>>> >> free space for 1 page, order 0
>>> >>
>>> >> #2 New allocation request of order 1 (2 pages) comes, new block is allocated
>>> >> since we do not have free space to complete this request. New block is put
>>> >> into a head of a free list:
>>> >>
>>> >> HEAD |----------|xxxxxxxxx-| TAIL
>>> >>
>>> >> #3 Two pages were occupied in a new found block:
>>> >>
>>> >> HEAD |xx--------|xxxxxxxxx-| TAIL
>>> >> ^
>>> >> two pages mapped here
>>> >>
>>> >> #4 New allocation request of order 0 (1 page) comes. Block, which was created
>>> >> on #2 step, is located at the beginning of a free list, so it will be found
>>> >> first:
>>> >>
>>> >> HEAD |xxX-------|xxxxxxxxx-| TAIL
>>> >> ^ ^
>>> >> page mapped here, but better to use this hole
>>> >>
>>> >> It is obvious, that it is better to complete request of #4 step using the old
>>> >> block, where free space is left, because in other case fragmentation will be
>>> >> highly increased.
>>> >>
>>> >> But fragmentation is not only the case. The most worst thing is that I can
>>> >> easily create scenario, when the whole vmalloc space is exhausted by blocks,
>>> >> which are not used, but already dirty and have several free pages.
>>> >>
>>> >> Let's consider this function which execution should be pinned to one CPU:
>>> >>
>>> >> ------------------------------------------------------------------------------
>>> >> /* Here we consider that our block is equal to 1MB, thus 256 pages */
>>> >> static void exhaust_virtual_space(struct page *pages[256], int iters)
>>> >> {
>>> >> /* Firstly we have to map a big chunk, e.g. 16 pages.
>>> >> * Then we have to occupy the remaining space with smaller
>>> >> * chunks, i.e. 8 pages. At the end small hole should remain.
>>> >> * So at the end of our allocation sequence block looks like
>>> >> * this:
>>> >> * XX big chunk
>>> >> * |XXxxxxxxx-| x small chunk
>>> >> * - hole, which is enough for a small chunk,
>>> >> * but not for a big chunk
>>> >> */
>>> >> unsigned big_allocs = 1;
>>> >> /* -1 for hole, which should be left at the end of each block
>>> >> * to keep it partially used, with some free space available */
>>> >> unsigned small_allocs = (256 - 16) / 8 - 1;
>>> >> void *vaddrs[big_allocs + small_allocs];
>>> >>
>>> >> while (iters--) {
>>> >> int i = 0, j;
>>> >>
>>> >> /* Map big chunk */
>>> >> vaddrs[i++] = vm_map_ram(pages, 16, -1, PAGE_KERNEL);
>>> >>
>>> >> /* Map small chunks */
>>> >> for (j = 0; j < small_allocs; j++)
>>> >> vaddrs[i++] = vm_map_ram(pages + 16 + j * 8, 8, -1,
>>> >> PAGE_KERNEL);
>>> >>
>>> >> /* Unmap everything */
>>> >> while (i--)
>>> >> vm_unmap_ram(vaddrs[i], (i ? 8 : 16));
>>> >> }
>>> >> }
>>> >> ------------------------------------------------------------------------------
>>> >>
>>> >> On every iteration new block (1MB of vm area in my case) will be allocated and
>>> >> then will be occupied, without attempt to resolve small allocation request
>>> >> using previously allocated blocks in a free list.
>>> >>
>>> >> In current patch I simply put newly allocated block to the tail of a free list,
>>> >> thus reduce fragmentation, giving a chance to resolve allocation request using
>>> >> older blocks with possible holes left.
>>> >
>>> > Hello,
>>> >
>>> > I think that if you put newly allocated block to the tail of a free
>>> > list, below example would results in enormous performance degradation.
>>> >
>>> > new block: 1MB (256 pages)
>>> >
>>> > while (iters--) {
>>> > vm_map_ram(3 or something else not dividable for 256) * 85
>>> > vm_unmap_ram(3) * 85
>>> > }
>>> >
>>> > On every iteration, it needs newly allocated block and it is put to the
>>> > tail of a free list so finding it consumes large amount of time.
>>> >
>>> > Is there any other solution to prevent your problem?
>>>
>>> Hello.
>>>
>>> My second patch fixes this problem.
>>> I occupy the block on allocation and avoid jumping to the search loop.
>>
>> I'm not sure that this fixes above case.
>> 'vm_map_ram (3) * 85' means 85 times vm_map_ram() calls.
>>
>> First vm_map_ram(3) caller could get benefit from your second patch.
>> But, second caller and the other callers in each iteration could not
>> get benefit and should iterate whole list to find suitable free block,
>> because this free block is put to the tail of the list. Am I missing
>> something?
>
> You are missing the fact that we occupy blocks in 2^n.
> So in your example 4 page slots will be occupied (order is 2), not 3.
Ah... Okay. I understand now.
> The maximum size of allocation is 32 pages for 32-bit system
> (if you try to map more, original alloc_vmap_area will be called).
>
> So the maximum order is 5. That means that worst case, before we make
> the decision
> to allocate new block, is to iterate 6 blocks:
>
> HEAD
> 1st block - has 1 page slot free (order 0)
> 2nd block - has 2 page slots free (order 1)
> 3rd block - has 4 page slots free (order 2)
> 4th block - has 8 page slots free (order 3)
> 5th block - has 16 page slots free (order 4)
> 6th block - has 32 page slots free (order 5)
> TAIL
>
> So the worst scenario is that each CPU queue can have 6 blocks in a free list.
Okay.
> This can happen only and only if you allocate blocks increasing the order.
> (as I did in the function written in the comment of the first patch)
> This is weird and rare case, but still it is possible.
> Afterwards you will get 6 blocks in a list.
>
> All further requests should be placed in a newly allocated block or
> some free slots
> should be found in a free list. Seems it does not look dramatically awful.
I think so, too.
Acked-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Thanks.
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