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NVMe commands with over 8 KB of discontiguous data allocate PRP list pages from the per-nvme_device dma_pool prp_page_pool or prp_small_pool. Each call to dma_pool_alloc() and dma_pool_free() takes the per-dma_pool spinlock. These device-global spinlocks are a significant source of contention when many CPUs are submitting to the same NVMe devices. On a workload issuing 32 KB reads from 16 CPUs (8 hypertwin pairs) across 2 NUMA nodes to 23 NVMe devices, we observed 2.4% of CPU time spent in _raw_spin_lock_irqsave called from dma_pool_alloc and dma_pool_free. Ideally, the dma_pools would be per-hctx to minimize contention. But that could impose considerable resource costs in a system with many NVMe devices and CPUs. As a compromise, allocate per-NUMA-node PRP list DMA pools. Map each nvme_queue to the set of DMA pools corresponding to its device and its hctx's NUMA node. This reduces the _raw_spin_lock_irqsave overhead by about half, to 1.2%. Preventing the sharing of PRP list pages across NUMA nodes also makes them cheaper to initialize. Link: https://lore.kernel.org/linux-nvme/CADUfDZqa=OOTtTTznXRDmBQo1WrFcDw1hBA7XwM7hzJ-hpckcA@mail.gmail.com/T/#u Signed-off-by: Caleb Sander Mateos <csander@purestorage.com> Reviewed-by: Jens Axboe <axboe@kernel.dk> Reviewed-by: Keith Busch <kbusch@kernel.org> Reviewed-by: Sagi Grimberg <sagi@grimberg.me> Reviewed-by: Kanchan Joshi <joshi.k@samsung.com> Signed-off-by: Christoph Hellwig <hch@lst.de> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.