Re: [PATCH 11/11] btrfs: add zstd compression level support

[David Sterba]

On Mon, Jan 28, 2019 at 04:24:37PM -0500, Dennis Zhou wrote:
> As mentioned above, a requirement that differs zstd from zlib is that
> higher levels of compression require more memory. To manage this, each
> compression level has its own queue of workspaces. A global LRU is used
> to help with reclaim. To guarantee forward progress, a max level
> workspace is preallocated and hidden from the LRU.
> 
> When getting a workspace, it uses a bitmap to identify the levels that
> are populated and scans up. If it finds a workspace that is greater than
> it, it uses it, but does not update the last_used time and the
> corresponding place in the LRU. This provides a mechanism to decrease
> memory utilization as we only keep around workspaces that are sized
> appropriately for the in use compression levels.
> 
> By knowing which compression levels have available workspaces, we can
> recycle rather than always create new workspaces as well as take
> advantage of the preallocated max level for forward progress. If we hit
> memory pressure, we sleep on the max level workspace. We continue to
> rescan in case we can use a smaller workspace, but eventually should be
> able to obtain the max level workspace or allocate one again should
> memory pressure subside. The memory requirement for decompression is the
> same as level 1, and therefore can use any of available workspace.
> 
> The number of workspaces is bound by an upper limit of the workqueue's
> limit which currently is 2 (percpu limit). Second, a reclaim timer is
> used to free inactive/improperly sized workspaces. The reclaim timer is
> set to 67s to avoid colliding with transaction commit (every 30s) and
> attempts to reclaim any unused workspace older than 45s.

> --- a/fs/btrfs/zstd.c
> +++ b/fs/btrfs/zstd.c
>  #define ZSTD_BTRFS_MAX_WINDOWLOG 17
>  #define ZSTD_BTRFS_MAX_INPUT (1 << ZSTD_BTRFS_MAX_WINDOWLOG)
>  #define ZSTD_BTRFS_DEFAULT_LEVEL 3
> +#define ZSTD_BTRFS_MAX_LEVEL 15
> +#define ZSTD_BTRFS_RECLAIM_NS (45 * NSEC_PER_SEC)
> +/* 67s to avoid clashing with transaction commit (every 30s) */
> +#define ZSTD_BTRFS_RECLAIM_JIFFIES (67 * HZ)

I think you can copy the relevant part of changelog here to explain the
logic of the levels and reclaim as it's scattered over several
functions. The description would give a nice overview in one place.