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Currently ext4_writepages() submits all pages with transaction started. When no page needs block allocation or extent conversion we can submit all dirty pages in the inode while holding a single transaction handle and when device is congested this can take significant amount of time. Thus ext4_writepages() can block transaction commits for extended periods of time. Take for example a simple benchmark simulating PostgreSQL database (pgioperf in mmtest). The benchmark runs 16 processes doing random reads from a huge file, one process doing random writes to the huge file, and one process doing sequential writes to a small files and frequently running fsync. With unpatched kernel transaction commits take on average ~18s with standard deviation of ~41s, top 5 commit times are: 274.466639s, 126.467347s, 86.992429s, 34.351563s, 31.517653s. After this patch transaction commits take on average 0.1s with standard deviation of 0.15s, top 5 commit times are: 0.563792s, 0.519980s, 0.509841s, 0.471700s, 0.469899s [ Modified so we use an explicit do_map flag instead of relying on io_end not being allocated, the since io_end->inode is needed for I/O error handling. -- tytso ] Signed-off-by: Jan Kara <jack@suse.cz> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.