Re: mm/zswap: change to writethrough

[Dan Streetman <ddstreet@ieee.org>]

On Tue, Nov 12, 2013 at 4:11 AM, Bob Liu <bob.liu@oracle.com> wrote:
>
> On 11/12/2013 03:12 AM, Dan Streetman wrote:
>> Seth, have you (or anyone else) considered making zswap a writethrough
>> cache instead of writeback?  I think that it would significantly help
>> the case where zswap fills up and starts writing back its oldest pages
>> to disc - all the decompression work would be avoided since zswap
>> could just evict old pages and forget about them, and it seems likely
>> that when zswap is full that's probably the worst time to add extra
>> work/delay, while adding extra disc IO (presumably using dma) before
>> zswap is full doesn't seem to me like it would have much impact,
>> except in the case where zswap isn't full but there is so little free
>> memory that new allocs are waiting on swap-out.
>>
>> Besides the additional disc IO that obviously comes with making zswap
>> writethrough (additional only before zswap fills up), are there any
>> other disadvantages?  Is it a common situation for there to be no
>> memory left and get_free_page actively waiting on swap-out, but before
>> zswap fills up?
>>
>> Making it writethrough also could open up other possible improvements,
>> like making the compression and storage of new swap-out pages async,
>> so the compression doesn't delay the write out to disc.
>>
>
> I like this idea and those benefits, the only question I'm not sure is
> would it be too complicate to implement this feature? It sounds like we
> need to reimplement something like swapcache to handle zswap write through.

Simply converting to writethrough should be as easy as returning
non-zero from zswap_frontswap_store(), although
zswap_writeback_entry() also needs simplification to skip the
writeback.  I think it shouldn't be difficult; I'll start working on a
first pass of a patch.

Thanks!

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