[dm-devel] [patch 4/4] dm-writecache: use new API for flushing

[Dan Williams]

On Wed, May 30, 2018 at 6:07 AM, Mikulas Patocka <mpatocka at redhat.com> wrote:
>
>
> On Mon, 28 May 2018, Dan Williams wrote:
>
>> On Mon, May 28, 2018 at 6:32 AM, Mikulas Patocka <mpatocka at redhat.com> wrote:
>> >
>> >
>> > On Sat, 26 May 2018, Dan Williams wrote:
>> >
>> >> On Sat, May 26, 2018 at 12:02 AM, Mikulas Patocka <mpatocka at redhat.com> wrote:
>> >> >
>> >> >
>> >> > On Fri, 25 May 2018, Dan Williams wrote:
>> >> >
>> >> >> On Fri, May 25, 2018 at 5:51 AM, Mike Snitzer <snitzer at redhat.com> wrote:
>> >> >> > On Fri, May 25 2018 at  2:17am -0400,
>> >> >> > Mikulas Patocka <mpatocka at redhat.com> wrote:
>> >> >> >
>> >> >> >> On Thu, 24 May 2018, Dan Williams wrote:
>> >> >> >>
>> >> >> >> > I don't want to grow driver-local wrappers for pmem. You should use
>> >> >> >> > memcpy_flushcache directly() and if an architecture does not define
>> >> >> >> > memcpy_flushcache() then don't allow building dm-writecache, i.e. this
>> >> >> >> > driver should 'depends on CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE'. I don't
>> >> >> >> > see a need to add a standalone flush operation if all relevant archs
>> >> >> >> > provide memcpy_flushcache(). As for commit, I'd say just use wmb()
>> >> >> >> > directly since all archs define it. Alternatively we could introduce
>> >> >> >> > memcpy_flushcache_relaxed() to be the un-ordered version of the copy
>> >> >> >> > routine and memcpy_flushcache() would imply a wmb().
>> >> >> >>
>> >> >> >> But memcpy_flushcache() on ARM64 is slow.
>> >> >>
>> >> >> Right, so again, what is wrong with memcpy_flushcache_relaxed() +
>> >> >> wmb() or otherwise making memcpy_flushcache() ordered. I do not see
>> >> >> that as a trailblazing requirement, I see that as typical review and a
>> >> >> reduction of the operation space that you are proposing.
>> >> >
>> >> > memcpy_flushcache on ARM64 is generally wrong thing to do, because it is
>> >> > slower than memcpy and explicit cache flush.
>> >> >
>> >> > Suppose that you want to write data to a block device and make it
>> >> > persistent. So you send a WRITE bio and then a FLUSH bio.
>> >> >
>> >> > Now - how to implement these two bios on persistent memory:
>> >> >
>> >> > On X86, the WRITE bio does memcpy_flushcache() and the FLUSH bio does
>> >> > wmb() - this is the optimal implementation.
>> >> >
>> >> > But on ARM64, memcpy_flushcache() is suboptimal. On ARM64, the optimal
>> >> > implementation is that the WRITE bio does just memcpy() and the FLUSH bio
>> >> > does arch_wb_cache_pmem() on the affected range.
>> >> >
>> >> > Why is memcpy_flushcache() is suboptimal on ARM? The ARM architecture
>> >> > doesn't have non-temporal stores. So, memcpy_flushcache() is implemented
>> >> > as memcpy() followed by a cache flush.
>> >> >
>> >> > Now - if you flush the cache immediatelly after memcpy, the cache is full
>> >> > of dirty lines and the cache-flushing code has to write these lines back
>> >> > and that is slow.
>> >> >
>> >> > If you flush the cache some time after memcpy (i.e. when the FLUSH bio is
>> >> > received), the processor already flushed some part of the cache on its
>> >> > own, so the cache-flushing function has less work to do and it is faster.
>> >> >
>> >> > So the conclusion is - don't use memcpy_flushcache on ARM. This problem
>> >> > cannot be fixed by a better implementation of memcpy_flushcache.
>> >>
>> >> It sounds like ARM might be better off with mapping its pmem as
>> >> write-through rather than write-back, and skip the explicit cache
>> >
>> > I doubt it would perform well - write combining combines the writes into a
>> > larger segments - and write-through doesn't.
>> >
>>
>> Last I checked write-through caching does not disable write combining
>>
>> >> management altogether. You speak of "optimal" and "sub-optimal", but
>> >> what would be more clear is fio measurements of the relative IOPs and
>> >> latency profiles of the different approaches. The reason I am
>> >> continuing to push here is that reducing the operation space from
>> >> 'copy-flush-commit' to just 'copy' or 'copy-commit' simplifies the
>> >> maintenance long term.
>> >
>> > I measured it (with nvme backing store) and late cache flushing has 12%
>> > better performance than eager flushing with memcpy_flushcache().
>>
>> I assume what you're seeing is ARM64 over-flushing the amount of dirty
>> data so it becomes more efficient to do an amortized flush at the end?
>> However, that effectively makes memcpy_flushcache() unusable in the
>> way it can be used on x86. You claimed that ARM does not support
>> non-temporal stores, but it does, see the STNP instruction. I do not
>> want to see arch specific optimizations in drivers, so either
>> write-through mappings is a potential answer to remove the need to
>> explicitly manage flushing, or just implement STNP hacks in
>> memcpy_flushcache() like you did with MOVNT on x86.
>>
>> > 131836 4k iops - vs - 117016.
>>
>> To be clear this is memcpy_flushcache() vs memcpy + flush?
>
> I found out what caused the difference. I used dax_flush on the version of
> dm-writecache that I had on the ARM machine (with the kernel 4.14, because
> it is the last version where dax on ramdisk works) - and I thought that
> dax_flush flushes the cache, but it doesn't.
>
> When I replaced dax_flush with arch_wb_cache_pmem, the performance
> difference between early flushing and late flushing disappeared.
>
> So I think we can remove this per-architecture switch from dm-writecache.

Great find! Thanks for the due diligence. Feel free to add:

    Acked-by: Dan Williams <dan.j.williams at intel.com>

...on the reworks to unify ARM and x86.