Re: [PATCH v6 09/13] zram: split zcomp into separate R/W streams

[Hillf Danton]

On Fri, 23 Oct 2026 14:01:32 +0900 Sergey Senozhatsky wrote:
> On (26/10/22 19:11), Hillf Danton wrote:
> [..]
> > What I am confused is -- why decompression needs to run faster without boosting
> > compression up. Where is the compressed material?
> 
> In a typical embedded device (e.g. android) zram write is swap-out
> (direct, indirect kswapd, or backgroud service swapping out idle apps),
> users don't see those.  zram read, on the contrary, is what users
> see - because that's swap-in, read bursts when user brings back idle
> app (camera app, etc.) which needs to get back to foregound quickly.
> So, the read path is more important.
> 
> I tested the series against PE.
> 
Thanks for sharing data.

> readers vs readers (no writers) 3 runs
> 
>     BASE+PE     IOPS 1,449k  avg 3.48us  sd 155.6us  p99.99 26.8us  max 280,820us
>                 IOPS 1,435k  avg 3.69us  sd 322.9us  p99.99 24.2us  max 540,512us
>                 IOPS 1,459k  avg 3.46us  sd  92.8us  p99.99 25.5us  max 172,186us
>     PATCHED+PE  IOPS 1,457k  avg 3.31us  sd  15.9us  p99.99 25.5us  max   7,580us
>                 IOPS 1,475k  avg 3.28us  sd  14.6us  p99.99 24.5us  max   7,327us
>                 IOPS 1,463k  avg 3.32us  sd  17.2us  p99.99 28.5us  max   6,816us
> 
     BASE+PE     IOPS 1,449k
                 IOPS 1,435k
                 IOPS 1,459k
		+)----------
		      4,343
	       /3)----------
		      1,447

     PATCHED+PE  IOPS 1,457k
                 IOPS 1,475k
                 IOPS 1,463k
		+)----------
		      4,395
	       /3)----------
		      1,465

		(1,465 - 1,447) / 1,447 = 18 / 1,447 = 1%


> R/W stream split makes sense for reasons that we already mentioned.
> Readers don't need to be blocked on Writers - they don't compete for
> the same data.  And, moreover, readers don't need to be serialized at
> all for stateful decompression.
> 
> writers vs readers 3 runs
> 
>     BASE+PE     IOPS 1,271k  avg 5.57us  sd 1,393us  p99.99 27.5us  max 1,581,800us
>                 IOPS 1,461k  avg 3.43us  sd    78us  p99.99 23.9us  max   227,015us
>                 IOPS 1,444k  avg 3.48us  sd   222us  p99.99 23.9us  max   592,628us
>     PATCHED+PE  IOPS 1,457k  avg 3.32us  sd    18us  p99.99 27.5us  max     5,620us
>                 IOPS 1,464k  avg 3.31us  sd    17us  p99.99 24.2us  max     5,621us
>                 IOPS 1,475k  avg 3.29us  sd    16us  p99.99 26.5us  max     5,620us
> 
     BASE+PE     IOPS 1,271k
                 IOPS 1,461k
                 IOPS 1,444k
		+)----------
		      4,176
	       /3)----------
		      1,392

     PATCHED+PE  IOPS 1,457k
                 IOPS 1,464k
                 IOPS 1,475k
		+)----------
		      4,396
	       /3)----------
		      1,465

		(1,465 - 1,392) / 1,392 = 73 / 1,392 = 5%


	improvment	within 5%	more than 10%	more than 20%
	(mIngo's) rule	safely ignored	good		standing ovation

More than two miles away from standing ovation.