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1dc32a67e9
When doing a zfs send on a dataset with small recordsize (e.g. 8K), performance is dominated by the per-block overheads. This is especially true with `zfs send --compressed`, which further reduces the amount of data sent, for the same number of blocks. Several threads are involved, but the limiting factor is the `send_prefetch` thread, which is 100% on CPU. The main job of the `send_prefetch` thread is to issue zio's for the data that will be needed by the main thread. It does this by calling `arc_read(ARC_FLAG_PREFETCH)`. This has an immediate cost of creating an arc_hdr, which takes around 14% of one CPU. It also induces later costs by other threads: * Since the data was only prefetched, dmu_send()->dmu_dump_write() will need to call arc_read() again to get the data. This will have to look up the arc_hdr in the hash table and copy the data from the scatter ABD in the arc_hdr to a linear ABD in arc_buf. This takes 27% of one CPU. * dmu_dump_write() needs to arc_buf_destroy() This takes 11% of one CPU. * arc_adjust() will need to evict this arc_hdr, taking about 50% of one CPU. All of these costs can be avoided by bypassing the ARC if the data is not already cached. This commit changes `zfs send` to check for the data in the ARC, and if it is not found then we directly call `zio_read()`, reading the data into a linear ABD which is used by dmu_dump_write() directly. The performance improvement is best expressed in terms of how many blocks can be processed by `zfs send` in one second. This change increases the metric by 50%, from ~100,000 to ~150,000. When the amount of data per block is small (e.g. 2KB), there is a corresponding reduction in the elapsed time of `zfs send >/dev/null` (from 86 minutes to 58 minutes in this test case). In addition to improving the performance of `zfs send`, this change makes `zfs send` not pollute the ARC cache. In most cases the data will not be reused, so this allows us to keep caching useful data in the MRU (hit-once) part of the ARC. Reviewed-by: Paul Dagnelie <pcd@delphix.com> Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Matthew Ahrens <mahrens@delphix.com> Closes #10067 |
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| .. | ||
| os | ||
| sys | ||
| .gitignore | ||
| libnvpair.h | ||
| libuutil_common.h | ||
| libuutil_impl.h | ||
| libuutil.h | ||
| libzfs_core.h | ||
| libzfs_impl.h | ||
| libzfs.h | ||
| libzutil.h | ||
| Makefile.am | ||
| thread_pool.h | ||
| zfeature_common.h | ||
| zfs_comutil.h | ||
| zfs_deleg.h | ||
| zfs_fletcher.h | ||
| zfs_namecheck.h | ||
| zfs_prop.h | ||