[PATCH v4 2/2] erofs-utils: optimize buffer allocation logic
胡玮文
sehuww at mail.scut.edu.cn
Mon Jan 18 23:29:23 AEDT 2021
Hi Xiang,
Thank you, your patch is indeed clearer. Could you explain why you don’t like my added erofs_dbg? I found them very useful when debugging.
I’ve updated the commit message, with some fixes (see inline) in PATCH v5 (coming soon).
> 在 2021年1月16日，14:41，Gao Xiang <hsiangkao at aol.com> 写道：
>
> From: Hu Weiwen <sehuww at mail.scut.edu.cn>
>
> When using EROFS to pack our dataset which consists of millions of
> files, mkfs.erofs is very slow compared with mksquashfs.
>
> The bottleneck is `erofs_balloc` and `erofs_mapbh` function, which
> iterate over all previously allocated buffer blocks, making the
> complexity of the algrithm O(N^2) where N is the number of files.
>
> With this patch:
>
> * global `last_mapped_block` is mantained to avoid full scan in
> `erofs_mapbh` function.
>
> * global `non_full_buffer_blocks` mantains a list of buffer block for
> each type and each possible remaining bytes in the block. Then it is
> used to identify the most suitable blocks in future `erofs_balloc`,
> avoiding full scan.
>
> Some new data structure is allocated in this patch, more RAM usage is
> expected, but not much. When I test it with ImageNet dataset (1.33M
> files), 7GiB RAM is consumed, and it takes about 4 hours. Most time is
> spent on IO.
>
> Signed-off-by: Hu Weiwen <sehuww at mail.scut.edu.cn>
> Signed-off-by: Gao Xiang <hsiangkao at aol.com>
> ---
>
> I've simplified the most-fit finding logic of v3... Since buffers.off
> has to be aligned to alignsize, so I think it's better to use
> buffers.off as the index of mapped_buckets compared to using remaining
> size as it looks more straight-forward.
>
> Also, I found the exist logic handling expended blocks might be
> potential ineffective as well... we have to skip used < used0 only
> after oob (extra blocks is allocated, so not expend such blocks but
> allocate a new bb...) It might be more effective to reuse such
> non-mapped buffer blocks...
>
> Thanks,
> Gao Xiang
>
> include/erofs/cache.h | 1 +
> lib/cache.c | 91 +++++++++++++++++++++++++++++++++++++------
> 2 files changed, 81 insertions(+), 11 deletions(-)
>
> diff --git a/include/erofs/cache.h b/include/erofs/cache.h
> index f8dff67b9736..611ca5b8432b 100644
> --- a/include/erofs/cache.h
> +++ b/include/erofs/cache.h
> @@ -39,6 +39,7 @@ struct erofs_buffer_head {
>
> struct erofs_buffer_block {
> struct list_head list;
> + struct list_head mapped_list;
>
> erofs_blk_t blkaddr;
> int type;
> diff --git a/lib/cache.c b/lib/cache.c
> index 32a58311f563..a44e140bc77b 100644
> --- a/lib/cache.c
> +++ b/lib/cache.c
> @@ -18,6 +18,11 @@ static struct erofs_buffer_block blkh = {
> };
> static erofs_blk_t tail_blkaddr;
>
> +/* buckets for all mapped buffer blocks to boost up allocation */
> +static struct list_head mapped_buckets[2][EROFS_BLKSIZ];
> +/* last mapped buffer block to accelerate erofs_mapbh() */
> +static struct erofs_buffer_block *last_mapped_block = &blkh;
> +
> static bool erofs_bh_flush_drop_directly(struct erofs_buffer_head *bh)
> {
> return erofs_bh_flush_generic_end(bh);
> @@ -62,12 +67,17 @@ struct erofs_bhops erofs_buf_write_bhops = {
> /* return buffer_head of erofs super block (with size 0) */
> struct erofs_buffer_head *erofs_buffer_init(void)
> {
> + int i, j;
> struct erofs_buffer_head *bh = erofs_balloc(META, 0, 0, 0);
>
> if (IS_ERR(bh))
> return bh;
>
> bh->op = &erofs_skip_write_bhops;
> +
> + for (i = 0; i < ARRAY_SIZE(mapped_buckets); i++)
> + for (j = 0; j < ARRAY_SIZE(mapped_buckets[0]); j++)
> + init_list_head(&mapped_buckets[i][j]);
> return bh;
> }
>
> @@ -132,20 +142,55 @@ struct erofs_buffer_head *erofs_balloc(int type, erofs_off_t size,
> struct erofs_buffer_block *cur, *bb;
> struct erofs_buffer_head *bh;
> unsigned int alignsize, used0, usedmax;
> + unsigned int used_before, used;
>
> int ret = get_alignsize(type, &type);
>
> if (ret < 0)
> return ERR_PTR(ret);
> +
> + DBG_BUGON(type < 0 || type > META);
> alignsize = ret;
>
> used0 = (size + required_ext) % EROFS_BLKSIZ + inline_ext;
> usedmax = 0;
> bb = NULL;
>
> - list_for_each_entry(cur, &blkh.list, list) {
> - unsigned int used_before, used;
> + if (!used0 || alignsize == EROFS_BLKSIZ)
> + goto alloc;
> +
> + /* try to find a most-fit mapped buffer block first */
> + for (used_before = EROFS_BLKSIZ; used_before > 1; ) {
I would argue that it is more efficient if we use a more specific initial value for used_before.
> + struct list_head *bt = mapped_buckets[type] + --used_before;
> +
> + if (list_empty(bt))
> + continue;
> + cur = list_first_entry(bt, struct erofs_buffer_block,
> + mapped_list);
> +
> + /* last mapped block can be expended, don't handle it here */
> + if (cur == last_mapped_block)
> + continue;
> +
> + ret = __erofs_battach(cur, NULL, size, alignsize,
> + required_ext + inline_ext, true);
> + if (ret < 0)
If used_before is chosen properly, this should never fail.
> + continue;
> +
> + /* should contain all data in the current block */
> + used = ret + required_ext + inline_ext;
> + DBG_BUGON(used > EROFS_BLKSIZ);
> +
> + bb = cur;
> + usedmax = used;
> + break;
> + }
>
> + /* try to start from the last mapped one, which can be expended */
> + cur = last_mapped_block;
> + if (cur == &blkh)
> + cur = list_next_entry(cur, list);
> + for (; cur != &blkh; cur = list_next_entry(cur, list)) {
> used_before = cur->buffers.off % EROFS_BLKSIZ;
>
> /* skip if buffer block is just full */
> @@ -187,6 +232,7 @@ struct erofs_buffer_head *erofs_balloc(int type, erofs_off_t size,
> goto found;
> }
>
> +alloc:
> /* allocate a new buffer block */
> if (used0 > EROFS_BLKSIZ)
> return ERR_PTR(-ENOSPC);
> @@ -200,6 +246,7 @@ struct erofs_buffer_head *erofs_balloc(int type, erofs_off_t size,
> bb->buffers.off = 0;
> init_list_head(&bb->buffers.list);
> list_add_tail(&bb->list, &blkh.list);
> + init_list_head(&bb->mapped_list);
>
> bh = malloc(sizeof(struct erofs_buffer_head));
> if (!bh) {
> @@ -214,6 +261,18 @@ found:
> return bh;
> }
>
> +static void erofs_bupdate_mapped(struct erofs_buffer_block *bb)
> +{
> + struct list_head *bkt;
> +
> + if (bb->blkaddr == NULL_ADDR)
> + return;
> +
> + bkt = mapped_buckets[bb->type] + bb->buffers.off % EROFS_BLKSIZ;
> + list_del(&bb->mapped_list);
> + list_add_tail(&bb->mapped_list, bkt);
> +}
> +
> struct erofs_buffer_head *erofs_battach(struct erofs_buffer_head *bh,
> int type, unsigned int size)
> {
> @@ -239,6 +298,7 @@ struct erofs_buffer_head *erofs_battach(struct erofs_buffer_head *bh,
> free(nbh);
> return ERR_PTR(ret);
> }
> + erofs_bupdate_mapped(bb);
This line should goes into “__erofs_battach()”? Since “erofs_balloc()” only calls that function.
> return nbh;
>
> }
> @@ -247,8 +307,11 @@ static erofs_blk_t __erofs_mapbh(struct erofs_buffer_block *bb)
> {
> erofs_blk_t blkaddr;
>
> - if (bb->blkaddr == NULL_ADDR)
> + if (bb->blkaddr == NULL_ADDR) {
> bb->blkaddr = tail_blkaddr;
> + last_mapped_block = bb;
> + erofs_bupdate_mapped(bb);
> + }
>
> blkaddr = bb->blkaddr + BLK_ROUND_UP(bb->buffers.off);
> if (blkaddr > tail_blkaddr)
> @@ -259,15 +322,16 @@ static erofs_blk_t __erofs_mapbh(struct erofs_buffer_block *bb)
>
> erofs_blk_t erofs_mapbh(struct erofs_buffer_block *bb)
> {
> - struct erofs_buffer_block *t, *nt;
> + struct erofs_buffer_block *t = last_mapped_block;
>
> - if (!bb || bb->blkaddr == NULL_ADDR) {
> - list_for_each_entry_safe(t, nt, &blkh.list, list) {
> - (void)__erofs_mapbh(t);
> - if (t == bb)
> - break;
> - }
> - }
> + do {
> + t = list_next_entry(t, list);
> + if (t == &blkh)
> + break;
> +
> + DBG_BUGON(t->blkaddr != NULL_ADDR);
> + (void)__erofs_mapbh(t);
> + } while (t != bb);
> return tail_blkaddr;
> }
>
> @@ -309,6 +373,7 @@ bool erofs_bflush(struct erofs_buffer_block *bb)
>
> erofs_dbg("block %u to %u flushed", p->blkaddr, blkaddr - 1);
>
> + list_del(&p->mapped_list);
> list_del(&p->list);
> free(p);
> }
> @@ -332,6 +397,10 @@ void erofs_bdrop(struct erofs_buffer_head *bh, bool tryrevoke)
> if (!list_empty(&bb->buffers.list))
> return;
>
> + if (bb == last_mapped_block)
> + last_mapped_block = list_prev_entry(bb, list);
> +
> + list_del(&bb->mapped_list);
> list_del(&bb->list);
> free(bb);
>
> --
> 2.24.0
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