Massive Solaris Scalability for the T5-8 and M5-32, Part 3
By Steve Sistare on Apr 10, 2013
The Solaris thread scheduler is little changed, as the architecture of balancing runnable threads across levels in the processor resource hierarchy , which I described when the T2 processor was introduced, has scaled well. However, we have continued to optimize the clock function of the scheduler. Clock is responsible for quanta expiration, timeout processing, resource accounting for every CPU, and for misc housekeeping functions. Previously, we parallelized quanta expiration and timeout expiration(aka callouts). In Solaris 11, we eliminated the need to acquire the process and thread locks in most cases during quanta expiration and accounting, and we eliminated or reduced the impact of several smallish O(N) calculations that had become significant at 1536 CPUs. The net result is that all functionality associated with clock scales nicely, and CPU 0 does not accumulate noticeable %sys CPU time due to clock processing.
SPARC systems use an IOMMU to map PCI-E virtual addresses to physical memory. The PCI VA space is a limited resource with high demand. The VA span is only 2GB to maintain binary compatibility with traditional DDI functions, and many drivers pre-map large DMA buffer pools so that mapping is not on the critical path for transport operations. Every CPU can post concurrent DMA requests, thus demand increases with scale. Managing these conflicting demands is a challenge. We reimplemented DVMA allocation using the Solaris kmem_cache and vmem facilities, with object size and quanta chosen to match common DMA transfer sizes. This provides a good balance between contention-free per-CPU caching, and redistribution of free space in the back end magazine and slab layers. We also modified drivers to use DMA pools more efficiently, and we modified the IOMMU code so that 2GB of VA is available per PCI function, rather than per PCI root port.
The net result for the end user is higher device throughput and/or lower CPU utilization per unit of throughput on larger systems.
The very tools we use to analyze scalability may exhibit problems themselves, because they must collect data for all the entities on a system. We noticed that mpstat was consuming so much CPU time on large systems that it could not sample at 1 second intervals and was falling behind. mpstat collects data for all CPUs in every interval, but 1536 CPUs is not a large number to handle in 1 second, so something was amiss. Profiling showed the time was spent searching for per-cpu kstats (see kstat(3KSTAT)), and every lookup searched the entire kc_chain linked list of all kstats. Since the number of kstats grows with NCPU, the overall algorithm takes time O(NCPU^2), which explodes on the larger systems. We modified the kstat library to build a hash table when kstats are opened, and re-implemented kstat_lookup() on that. This reduced cpu consumption by 8X on our "small" 512-CPU test system, and improves the performance of all tools that are based on libkstat, including mpstat, vmstat, iostat, and sar.
Even dtrace is not immune. When a script starts, dtrace allocates multi-megabyte trace buffers for every CPU in the domain, using a single thread, and frees the buffers on script termination using a single thread. On a T3-4 with 512 CPUs, it took 30 seconds to run a null D script. Even worse, the allocation is done while holding the global cpu_lock, which serializes the startup of other D scripts, and causes long pauses in the output of some stat commands that briefly take cpu_lock while sampling. We fixed this in Solaris 11.1 by allocating and freeing the trace buffers in parallel using vmtasks, and by hoisting allocation out of the cpu_lock critical path.
Large scale can impact the usability of a tool. Some stat tools produce a row of output per CPU in every sampling interval, making it hard to spot important clues in the torrent of data. In Solaris 11.1, we provide new aggregation and sorting options for the mpstat, cpustat, and trapstat commands that allow the user to make sense of the data. For example, the command
mpstat -k intr -A 4 -m 10 5sorts CPUs by the interrupts metric, partitions them into quartiles, and aggregates each quartile into a single row by computing the mean column values within each. See the man pages for details.
Large servers take longer to reboot than small servers. Why? They must initialize more CPUs, memory, and devices, but much of the shutdown and startup code in firmware and the kernel is single threaded. We are addressing that. On shutdown, Solaris now scans memory in parallel to look for dirty pages that must be flushed to disk. The sun4v hypervisor zero's a domain's memory in parallel, using CPUs that are physically closest to memory for maximum bandwidth. On startup, Solaris VM initializes per-page metadata using SPARC cache initializing block stores, which speeds metadata initialization by more than 2X. We also fixed an O(NCPU^2) algorithm in bringing CPUs online, and an O(NCPU) algorithm in reclaiming memory from firmware. In total, we have reduced the reboot time for M5-32 systems by many minutes, and we continue to work on optimizations in this area.
In these few short posts, I have summarized the work of many people over a period of years that has pushed Solaris to new heights of scalability, and I look forward to seeing what our customers will do with their massive T5-8 and M5-32 systems. However, if you have seen the SPARC processor roadmap, you know that our work is not done. Onward and upward!