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    SubjectRe: [PATCHv4 2/9] zram: Add recompression algorithm sysfs knob
    On Sat, Nov 05, 2022 at 09:00:33AM +0900, Sergey Senozhatsky wrote:
    > On (22/11/05 08:41), Sergey Senozhatsky wrote:
    > > One can be SW one can be HW. So I thought about having flexibility here.
    > > Instead of doing
    > >
    > > for (idx = 1; idx < MAX_IDX; idx++) {
    > > len = zcomp_compress(zram->comps[idx]);
    > > if (len <= threshold)
    > > break;
    > > }
    > >
    > > We would just directly use the suggested algo.
    > >
    > > But we probably don't need that param at all and can use
    > > the loop instead?
    >
    > My idea was that recompress does not loop through the algos (what
    > on the fly recompression can do for instance), but instead
    > recompress only does one thing.
    >
    > Because we can have, for instance, something like this
    >
    > algo=zstd priority=1
    > algo=deflate priority=2
    >
    > And we recompress with algo=zstd only first. Without going
    > into the slowest, most CPU and power consuming one. If the
    > memory pressure keeps increasing then we might do algo=deflate
    > recompress, as the last resort before we do writeback. But we
    > may skip algo=deflate altogether and go straight to writeback,
    > for instance because we have less than 30% of battery left.
    >
    > So the reason I suggested algo= in recompress was, basically,
    > for that for having exact control of what recompression does.


    I am thinking like this:

    * Without recomp_algo setup, user can do whatever they want on the fly


    echo "type=idle threshold=3000 algo=zstd" > recompress

    Later they could do

    echo "type=idle threshold=3000 algo=deflate" > recompress

    or

    writeback to backing device

    * With recomp_algo setup like this,

    echo "algo=zstd priority=1" > recomp_algo
    echo "algo=deflate priority=2" > recomp_algo
    ..
    ..
    echo "type=idle threshold=3000" > recompress

    If zstd fails, it will continue to work with deflate transparently.

    IOW, "algo=" in *recompress* interface will override the global policy
    in the *recomp_algo* setup. Otherwise, the recomp_algo's set up will
    work.

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