On Wed, Jan 15, 2020 at 11:45 AM Ira Weiny <ira.weiny@xxxxxxxxx> wrote: > > On Wed, Jan 15, 2020 at 09:38:34AM -0800, Darrick J. Wong wrote: > > On Wed, Jan 15, 2020 at 12:37:15PM +0100, Jan Kara wrote: > > > On Fri 10-01-20 11:29:31, ira.weiny@xxxxxxxxx wrote: > > > > From: Ira Weiny <ira.weiny@xxxxxxxxx> > > > > > > > > In order for users to determine if a file is currently operating in DAX > > > > mode (effective DAX). Define a statx attribute value and set that > > > > attribute if the effective DAX flag is set. > > > > > > > > To go along with this we propose the following addition to the statx man > > > > page: > > > > > > > > STATX_ATTR_DAX > > > > > > > > DAX (cpu direct access) is a file mode that attempts to minimize > > > > "..is a file I/O mode"? > > or "... is a file state ..."? > > > > > software cache effects for both I/O and memory mappings of this > > > > file. It requires a capable device, a compatible filesystem > > > > block size, and filesystem opt-in. > > > > "...a capable storage device..." > > Done > > > > > What does "compatible fs block size" mean? How does the user figure out > > if their fs blocksize is compatible? Do we tell users to refer their > > filesystem's documentation here? > > Perhaps it is wrong for this to be in the man page at all? Would it be better > to assume the file system and block device are already configured properly by > the admin? > > For which the blocksize restrictions are already well documented. ie: > > https://www.kernel.org/doc/Documentation/filesystems/dax.txt > > ? > > How about changing the text to: > > It requires a block device and file system which have been configured > to support DAX. > > ? The goal was to document the gauntlet of checks that __generic_fsdax_supported() performs so someone could debug "why am I not able to get dax operation?" > > > > > > > It generally assumes all > > > > accesses are via cpu load / store instructions which can > > > > minimize overhead for small accesses, but adversely affect cpu > > > > utilization for large transfers. > > > > Will this always be true for persistent memory? For direct-mapped pmem there is no opportunity to do dma offload so it will always be true that application dax access consumes cpu to do I/O where something like NVMe does not. There has been unfruitful to date experiments with the driver using an offload engine for kernel internal I/O, but if you're use case is kernel internal I/O bound then you don't need dax. > > I'm not clear. Did you mean; "this" == adverse utilization for large transfers? > > > > > I wasn't even aware that large transfers adversely affected CPU > > utilization. ;) > > Sure vs using a DMA engine for example. Right, this is purely a statement about cpu memcpy vs device-dma. > > > > > > > File I/O is done directly > > > > to/from user-space buffers. While the DAX property tends to > > > > result in data being transferred synchronously it does not give > > > > "...transferred synchronously, it does not..." > > done. > > > > > > > the guarantees of synchronous I/O that data and necessary > > > > "...it does not guarantee that I/O or file metadata have been flushed to > > the storage device." > > The lack of guarantee here is mainly regarding metadata. > > How about: > > While the DAX property tends to result in data being transferred > synchronously, it does not give the same guarantees of > synchronous I/O where data and the necessary metadata are > transferred together. > > > > > > > metadata are transferred. Memory mapped I/O may be performed > > > > with direct mappings that bypass system memory buffering. > > > > "...with direct memory mappings that bypass kernel page cache." > > Done. > > > > > > > Again > > > > while memory-mapped I/O tends to result in data being > > > > I would move the sentence about "Memory mapped I/O..." to directly after > > the sentence about file I/O being done directly to and from userspace so > > that you don't need to repeat this statement. > > Done. > > > > > > > transferred synchronously it does not guarantee synchronous > > > > metadata updates. A dax file may optionally support being mapped > > > > with the MAP_SYNC flag which does allow cpu store operations to > > > > be considered synchronous modulo cpu cache effects. > > > > How does one detect or work around or deal with "cpu cache effects"? I > > assume some sort of CPU cache flush instruction is what is meant here, > > but I think we could mention the basics of what has to be done here: > > > > "A DAX file may support being mapped with the MAP_SYNC flag, which > > enables a program to use CPU cache flush operations to persist CPU store > > operations without an explicit fsync(2). See mmap(2) for more > > information."? > > That sounds better. I like the reference to mmap as well. > > Ok I changed a couple of things as well. How does this sound? > > > STATX_ATTR_DAX > > DAX (cpu direct access) is a file mode that attempts to minimize s/mode/state/? > software cache effects for both I/O and memory mappings of this > file. It requires a block device and file system which have > been configured to support DAX. It may not require a block device in the future. > > DAX generally assumes all accesses are via cpu load / store > instructions which can minimize overhead for small accesses, but > may adversely affect cpu utilization for large transfers. > > File I/O is done directly to/from user-space buffers and memory > mapped I/O may be performed with direct memory mappings that > bypass kernel page cache. > > While the DAX property tends to result in data being transferred > synchronously, it does not give the same guarantees of > synchronous I/O where data and the necessary metadata are Maybe use "O_SYNC I/O" explicitly to further differentiate the 2 meanings of "synchronous" in this sentence? > transferred together. > > A DAX file may support being mapped with the MAP_SYNC flag, > which enables a program to use CPU cache flush operations to s/operations/instructions/ > persist CPU store operations without an explicit fsync(2). See > mmap(2) for more information. I think this also wants a reference to the Linux interpretation of platform "persistence domains" we were discussing that here [1], but maybe it should be part of a "pmem" manpage that can be referenced from this man page. [1]: http://lore.kernel.org/r/20200108064905.170394-1-aneesh.kumar@xxxxxxxxxxxxx
