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When attaching a cached device (a.k.a backing device) to a cache device, bch_sectors_dirty_init() is called to count dirty sectors and stripes (see what bcache_dev_sectors_dirty_add() does) on the cache device. When bcache_dev_sectors_dirty_add() is called, set_bit(stripe, d->full_dirty_stripes) or clear_bit(stripe, d->full_dirty_stripes) operation will always be performed. In full_dirty_stripes, each 1bit represents stripe_size (8192) sectors (512B), so 1bit=4MB (8192*512), and each CPU cache line=64B=512bit=2048MB. When 20 threads process a cached disk with 100G dirty data, a single thread processes about 23M at a time, and 20 threads total 460M. These full_dirty_stripes bits corresponding to the 460M data is likely to fall in the same CPU cache line. When one of these threads performs a set_bit or clear_bit operation, the same CPU cache line of other threads will become invalid and must read the full_dirty_stripes from the main memory again. Compared with single thread, the time of a bcache_dev_sectors_dirty_add() call is increased by about 50 times in our test (100G dirty data, 20 threads, bcache_dev_sectors_dirty_add() is called more than 20 million times). This patch tries to test_bit before set_bit or clear_bit operation. Therefore, a lot of force set and clear operations will be avoided, and most of bcache_dev_sectors_dirty_add() calls will only read CPU cache line. Signed-off-by: Mingzhe Zou <mingzhe.zou@easystack.cn> Signed-off-by: Coly Li <colyli@suse.de>
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 Restructured Text 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.