On Wed, Jun 24, 2020 at 6:22 AM Mike Snitzer <snit...@redhat.com> wrote: > > On Wed, Jun 24 2020 at 12:54am -0400, > Damien Le Moal <damien.lem...@wdc.com> wrote: > > > On 2020/06/24 0:23, Mike Snitzer wrote: > > > On Tue, Jun 23 2020 at 11:07am -0400, > > > Ignat Korchagin <ig...@cloudflare.com> wrote: > > > > > >> Do you think it may be better to break it in two flags: one for read > > >> path and one for write? So, depending on the needs and workflow these > > >> could be enabled independently? > > > > > > If there is a need to split, then sure. But I think Damien had a hard > > > requirement that writes had to be inlined but that reads didn't _need_ > > > to be for his dm-zoned usecase. Damien may not yet have assessed the > > > performance implications, of not have reads inlined, as much as you > > > have. > > > > We did do performance testing :) > > The results are mixed and performance differences between inline vs > > workqueues > > depend on the workload (IO size, IO queue depth and number of drives being > > used > > mostly). In many cases, inlining everything does really improve performance > > as > > Ignat reported. > > > > In our testing, we used hard drives and so focused mostly on throughput > > rather > > than command latency. The added workqueue context switch overhead and crypto > > work latency compared to typical HDD IO times is small, and significant > > only if > > the backend storage as short IO times. > > > > In the case of HDDs, especially for large IO sizes, inlining crypto work > > does > > not shine as it prevents an efficient use of CPU resources. This is > > especially > > true with reads on a large system with many drives connected to a single > > HBA: > > the softirq context decryption work does not lend itself well to using other > > CPUs that did not receive the HBA IRQ signaling command completions. The > > test > > results clearly show much higher throughputs using dm-crypt as is. > > > > On the other hand, inlining crypto work significantly improves workloads of > > small random IOs, even for a large number of disks: removing the overhead of > > context switches allows faster completions, allowing sending more requests > > to > > the drives more quickly, keeping them busy. > > > > For SMR, the inlining of write requests is *mandatory* to preserve the > > issuer > > write sequence, but encryption work being done in the issuer context > > (writes to > > SMR drives can only be O_DIRECT writes), efficient CPU resource usage can be > > achieved by simply using multiple writer thread/processes, working on > > different > > zones of different disks. This is a very reasonable model for SMR as writes > > into > > a single zone have to be done under mutual exclusion to ensure > > sequentiality. > > > > For reads, SMR drives are essentially exactly the same as regular disks, so > > as-is or inline are both OK. Based on our performance results, allowing the > > user > > to have the choice of inlining or not reads based on the target workload > > would > > be great. > > > > Of note is that zone append writes (emulated in SCSI, native with NVMe) are > > not > > subject to the sequential write constraint, so they can also be executed > > either > > inline or asynchronously. > > > > > So let's see how Damien's work goes and if he trully doesn't need/want > > > reads to be inlined then 2 flags can be created. > > > > For SMR, I do not need inline reads, but I do want the user to have the > > possibility of using this setup as that can provide better performance for > > some > > workloads. I think that splitting the inline flag in 2 is exactly what we > > want: > > > > 1) For SMR, the write-inline flag can be automatically turned on when the > > target > > device is created if the backend device used is a host-managed zoned drive > > (scsi > > or NVMe ZNS). For reads, it would be the user choice, based on the target > > workload. > > 2) For regular block devices, write-inline only, read-inline only or both > > would > > be the user choice, to optimize for their target workload. > > > > With the split into 2 flags, my SMR support patch becomes very simple. > > OK, thanks for all the context. Was a fun read ;) > > SO let's run with splitting into 2 flags. Ignat would you be up to > tweaking your patch to provide that and post a v2? > > An added bonus would be to consolidate your 0/1 and 1/1 patch headers, > and add in the additional answers you provided in this thread to help > others understand the patch (mainly some more detail about why tasklet > is used).
Yes, will do > Thanks, > Mike >
