Re: [PATCH v2 11/11] zram: use rw_semaphore for decompression streams

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (26/10/09 16:11), Sergey Senozhatsky wrote:
> Currently, per-CPU decompression streams use a mutex, so there is
> a chance for priority inversion - a low priority Reader can get
> preempted by a higher priority Reader, which simply will get
> blocked on the stream mutex.
> 
> Switch zcomp_dstrm lock from mutex to rw_semaphore to permit parallel
> usage.  However, things are not completely straightforward.  Many
> backends have stateless decompression and don't modify decompression
> context, so those can be used concurrently.  But then there are backends
> that do modify decompression context.  We also can have compressed
> object span across physical pages, which zcomp linearises using stream's
> ->local_copy buffer before decompression.  That buffer, clearly, cannot
> be used concurrently.
> 
> When the backend has no per-stream decompression context (stateless)
> and zsmalloc object fits in a single page (ZCOMP_DECOMP_INPLACE), take
> the stream lock in shared mode (read).  Otherwise, when ->local_copy
> is needed (ZCOMP_DECOMP_BOUNCE) or the backend has a decompression
> context (zstd, deflate), take the lock in exclusive mode (write).
> 
> A quick synthetic fio test on 24 CPUs (preempt=full, lz4, 48 low
> priority (nice 19) readers, 24 normal priority (nice 0) CPU hogs
> and 24 high priority (nice -19) readers):
> 
>   Metric           Base               Patched
>   ----------------------------------------------------------
>   Read IOPS        473k               680k          (+43.8%)
>   Read clat avg    35.98 us           21.28 us      (-40.9%)
>   Read clat p99.95 55.55 us           20.86 us      (-2.7x)
>   Read clat p99.99 6,717 us           2,834 us      (-2.4x)
>   Read clat max    2,005,100 us       1,614,300 us  (-19.5%)
> 
> Suggested-by: Brian Geffon <bgeffon@google.com>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>

JFI
I have completely reworked this patch.  Will run tests and then
send out v3.