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?
Thanks.
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