On Mon, May 18, 2020 at 11:32:04PM -0700, Darrick J. Wong wrote: > On Thu, May 14, 2020 at 08:34:54PM +1000, Dave Chinner wrote: > > > > Topic: SSD Optimised allocation policies > > > > Scope: > > Performance > > Storage efficiency > > > > Proposal: > > > > Non-rotational storage is typically very fast. Our allocation > > policies are all, fundamentally, based on very slow storage which > > has extremely high latency between IO to different LBA regions. We > > burn CPU to optimise for minimal seeks to minimise the expensive > > physical movement of disk heads and platter rotation. > > > > We know when the underlying storage is solid state - there's a > > "non-rotational" field in the block device config that tells us the > > storage doesn't need physical seek optimisation. We should make use > > of that. > > > > My proposal is that we look towards arranging the filesystem > > allocation policies into CPU-optimised silos. We start by making > > filesystems on SSDs with AG counts that are multiples of the CPU > > count in the system (e.g. 4x the number of CPUs) to drive > > I guess you and I have been doing this for years with seemingly few ill > effects. ;) > > That said, I did encounter a wackass system with 104 CPUs, a 1.4T RAID > array of spinning disks, 229 AGs sized ~6.5GB each, and a 50M log. The > ~900 io writers were sinking thesystem, so clearly some people are still > getting it wrong even with traditional storage. :( Unfortunately, we can't prevent people from screwing things up by not understanding what adverse impact twiddling mkfs knobs will have. And, realistically, that's way outside the scope of this topic... > > parallelism at the allocation level, and then associate allocation > > groups with specific CPUs in the system. Hence each CPU has a set of > > allocation groups is selects between for the operations that are run > > on it. Hence allocation is typically local to a specific CPU. > > Optimisation proceeds from the basis of CPU locality optimisation, > > not storage locality optimisation. > > I wonder how hard it would be to compile a locality map for storage and > CPUs from whatever numa and bus topology information the kernel already > knows about? We don't even need to do that. Just assigning specific AGs to specific CPUs will give per-cpu locality. And the thing is that this mapping can change from mount to mount without adversely impacting performance, because storage locality is largely irrelevant to performance. > > What this allows is processes on different CPUs to never contend for > > allocation resources. Locality of objects just doesn't matter for > > solid state storage, so we gain nothing by trying to group inodes, > > directories, their metadata and data physically close together. We > > want writes that happen at the same time to be physically close > > together so we aggregate them into larger IOs, but we really > > don't care about optimising write locality for best read performance > > (i.e. must be contiguous for sequential access) for this storage. > > > > Further, we can look at faster allocation strategies - we don't need > > to find the "nearest" if we don't have a contiguous free extent to > > allocate into, we just want the one that costs the least CPU to > > find. This is because solid state storage is so fast that filesystem > > performance is CPU limited, not storage limited. Hence we need to > > think about allocation policies differently and start optimising > > them for minimum CPU expenditure rather than best layout. > > > > Other things to discuss include: > > - how do we convert metadata structures to write-once style > > behaviour rather than overwrite in place? > > (Hm?) So we don't have to overwrite metadata in place. Overwrite in place is not nice to SSDs because they can't overwrite in place and this leads to write amplification in the device. e.g. I was wondering if there was ways we could look at changing the individual btrees to use something like a log structured merge tree... > > - extremely large block sizes for metadata (e.g. 4MB) to > > align better with SSD erase block sizes > > If we had metadata blocks that size, I'd advocate for studying how we > could restructure the btree to log updates in the slack space and only > checkpoint lower in the tree when necessary. well, I was kinda thinking that the "write once style" algorithms go fit much better with large blocks than update-in-place, especially with the way we log buffers right now. Blocks that large would allow storing deltas in the block, then when the block is full rewriting the entire block with all the deltas applied. And that's where write-once algorithms become interesting; we compress a portion of the tree and the new index into the block(s) and rewrite the root pointer... > > - what parts of the allocation algorithms don't we need > > Brian reworked part of the allocator a couple of cycles ago to reduce > the long tail latency of chasing through one free space btree when the > other one would have given it a quick answer; how beneficial has that > been? Could it be more aggressive? Yeah, that's what I'm getting at. All the "near/exact/size" stuff has a lot of overhead to find the best target. For SSDs "near" doesn't matter, so if we want a specific size we can jsut take the first fit. "exact" is only used to do contiguous allocation - that still has benefit from a filesystem persepective (less extents in data files) even if it doesn't improve IO performance, so I think there's whole reams of selection logic we can completely ignore for SSDs... > > - are we better off with huge numbers of small AGs rather > > than fewer large AGs? > > There's probably some point of dimininishing returns, but this seems > likely. Has anyone studied this recently? Not beyond "I need 100+ AGs on this 400GB SSD for allocation parallelism". i.e. I've typically just increased the number of AGs until nothing blocks on AGI or AGF headers, and ignored everything else. We probably need to look at how to handle thousands of AGs effectively. That is two-fold: a) managing the memory overhead of having that many active AGs and b) how we select AGs to allocate from. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
