Re: [PATCH V4 07/13] fs: Add locking for a dynamic address space operations state

[Jan Kara]

On Wed 26-02-20 08:46:42, Dan Williams wrote:
> On Wed, Feb 26, 2020 at 1:29 AM Jonathan Halliday
> <jonathan.halliday@xxxxxxxxxx> wrote:
> >
> >
> > Hi All
> >
> > I'm a middleware developer, focused on how Java (JVM) workloads can
> > benefit from app-direct mode pmem. Initially the target is apps that
> > need a fast binary log for fault tolerance: the classic database WAL use
> > case; transaction coordination systems; enterprise message bus
> > persistence and suchlike. Critically, there are cases where we use log
> > based storage, i.e. it's not the strict 'read rarely, only on recovery'
> > model that a classic db may have, but more of a 'append only, read many
> > times' event stream model.
> >
> > Think of the log oriented data storage as having logical segments (let's
> > implement them as files), of which the most recent is being appended to
> > (read_write) and the remaining N-1 older segments are full and sealed,
> > so effectively immutable (read_only) until discarded. The tail segment
> > needs to be in DAX mode for optimal write performance, as the size of
> > the append may be sub-block and we don't want the overhead of the kernel
> > call anyhow. So that's clearly a good fit for putting on a DAX fs mount
> > and using mmap with MAP_SYNC.
> >
> > However, we want fast read access into the segments, to retrieve stored
> > records. The small access index can be built in volatile RAM (assuming
> > we're willing to take the startup overhead of a full file scan at
> > recovery time) but the data itself is big and we don't want to move it
> > all off pmem. Which means the requirements are now different: we want
> > the O/S cache to pull hot data into fast volatile RAM for us, which DAX
> > explicitly won't do. Effectively a poor man's 'memory mode' pmem, rather
> > than app-direct mode, except here we're using the O/S rather than the
> > hardware memory controller to do the cache management for us.
> >
> > Currently this requires closing the full (read_write) file, then copying
> > it to a non-DAX device and reopening it (read_only) there. Clearly
> > that's expensive and rather tedious. Instead, I'd like to close the
> > MAP_SYNC mmap, then, leaving the file where it is, reopen it in a mode
> > that will instead go via the O/S cache in the traditional manner. Bonus
> > points if I can do it over non-overlapping ranges in a file without
> > closing the DAX mode mmap, since then the segments are entirely logical
> > instead of needing separate physical files.
> 
> Hi John,
> 
> IIRC we chatted about this at PIRL, right?
> 
> At the time it sounded more like mixed mode dax, i.e. dax writes, but
> cached reads. To me that's an optimization to optionally use dax for
> direct-I/O writes, with its existing set of page-cache coherence
> warts, and not a capability to dynamically switch the dax-mode.
> mmap+MAP_SYNC seems the wrong interface for this. This writeup
> mentions bypassing kernel call overhead, but I don't see how a
> dax-write syscall is cheaper than an mmap syscall plus fault. If
> direct-I/O to a dax capable file bypasses the block layer, isn't that
> about the maximum of kernel overhead that can be cut out of this use
> case? Otherwise MAP_SYNC is a facility to achieve efficient sub-block
> update-in-place writes not append writes.

Well, even for appends you'll pay the cost only once per page (or maybe even
once per huge page) when using MAP_SYNC. With a syscall you'll pay once per
write. So although it would be good to check real numbers, the design isn't
non-sensical to me.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR