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When we are attempting to start writeback for an existing extent in NOCOW mode, at run_delalloc_nocow(), we must check if the extent is shared, and if it is, fallback to a COW write. However we do such check while still holding a read lock on the leaf that contains the file extent item, and that check, the call to btrfs_cross_ref_exist(), can take some time because: 1) It needs to do a search on the extent tree, which obviously takes some time, specially if delayed references are being run at the moment, as we can block when trying to lock currently write locked btree nodes; 2) It needs to check the delayed references for any existing reference for our data extent, this requires acquiring the delayed references' spinlock and maybe block on the mutex of a delayed reference head in the case where there is a delayed reference for our data extent, in the worst case it makes us release the path on the extent tree and retry the whole process again (going back to step 1). There are other operations we do while holding the leaf locked that can take some significant time as well (specially all together): * btrfs_extent_readonly() - to check if the block group containing the extent is currently in RO mode. This requires taking a spinlock and searching for the block group in a rbtree that can be big on large filesystems; * csum_exist_in_range() - to search if there are any checksums in the csum tree for the extent. Like before, this can take some time if we are in a filesystem that has both COW and NOCOW files, in which case the csum tree is not empty; * btrfs_inc_nocow_writers() - increment the number of nocow writers in the block group that contains the data extent. Needs to acquire a spinlock and search for the block group in a rbtree that can be big on large filesystems. So just unlock the leaf (release the path) before doing all those checks, since we do not need it anymore. In case we can not do a NOCOW write for the extent, due to any of those checks failing, and the writeback range goes beyond that extents' length, we will do another btree search for the next file extent item. The following script that calls dbench was used to measure the impact of this change on a VM with 8 CPUs, 16Gb of ram, using a raw NVMe device directly (no intermediary filesystem on the host) and using a non-debug kernel (default configuration on Debian): $ cat test-dbench.sh #!/bin/bash DEV=/dev/sdk MNT=/mnt/sdk MOUNT_OPTIONS="-o ssd -o nodatacow" MKFS_OPTIONS="-m single -d single" mkfs.btrfs -f $MKFS_OPTIONS $DEV mount $MOUNT_OPTIONS $DEV $MNT dbench -D $MNT -t 300 64 umount $MNT Before this change: Operation Count AvgLat MaxLat ---------------------------------------- NTCreateX 9326331 0.317 399.957 Close 6851198 0.002 6.402 Rename 394894 2.621 402.819 Unlink 1883131 0.931 398.082 Deltree 256 19.160 303.580 Mkdir 128 0.003 0.016 Qpathinfo 8452314 0.068 116.133 Qfileinfo 1481921 0.001 5.081 Qfsinfo 1549963 0.002 4.444 Sfileinfo 759679 0.084 17.079 Find |
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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 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.