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
1dc32a67e9