On 10/12/2016 05:03 AM, Wang Xiaoguang wrote:
Since commit b02441999efcc6152b87cd58e7970bb7843f76cf, we don't wait all
ordered extents, but I run into some enospc errors when doing large file
create and delete tests, it's because shrink_delalloc() does not write
enough delalloc bytes and wait them finished:
    From: Miao Xie <mi...@cn.fujitsu.com>
    Date: Mon, 4 Nov 2013 23:13:25 +0800
    Subject: [PATCH] Btrfs: don't wait for the completion of all the ordered 
extents

    It is very likely that there are lots of ordered extents in the filesytem,
    if we wait for the completion of all of them when we want to reclaim some
    space for the metadata space reservation, we would be blocked for a long
    time. The performance would drop down suddenly for a long time.

Here we introduce a simple reclaim_priority variable, the higher the
value, the higher the priority, 0 is the minimum priority. The core
idea is:
        if (reclaim_priority >= 3)
                to_reclaim = 
percpu_counter_sum_positive(&root->fs_info->delalloc_bytes);
        else
                to_reclaim = orig * (2 << (reclaim_priority + 1));
As the priority increases, we try wo write more delalloc bytes, here orig
is the number of metadata we want to reserve. Meanwhile if
"reclaim_priority >= 3" returns true, we'll also wait all current ordered
extents to finish.

Ok this is closer to what I want but not quite there. I want to get rid of the magic numbers, so we start with

#define BTRFS_DEFAULT_FLUSH_PRIORITY 3

and once priority == 0 we know it's time to flush and wait for all the things. Then we use the priority as a multiplier, with DEFAULT being no multiplier, so it would look something like this

if (reclaim_priority)
        to_reclaim = orig * (2 << BTRFS_DEFAULT_FLUSH_PRIORITY - priority);
else
        to_reclaim = delalloc_bytes;
....
if (reclaim_priority)
        items_to_wait = calc_reclaim_items_nr(root, to_reclaim);
else
        items_to_wait = -1;


and then instead of

if (reclaim_priority > 3) {
        wake_all_tickets(&space_info->tickets);
...

we can change the while loop to be

while ((flush_state <= COMMIT_TRANS) || (--reclaim_priority >= 0));

and move the wake_all_tickets() to outside the loop, and add a

if (flush_state > COMMIT_TRANS)
        flush_state = FLUSH_DELAYED_ITEMS_NR;

to the start of the do loop.  Does that make sense?  Thanks,

Josef
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