Hi guys,
I have several questions on dm-thin when I'm testing and evaluating IO
performance of dm-thin. I would be grateful if someone could spend a
little time on it.
The first question is what's the purpose of data cell? In
thin_bio_map(), normal bio will be packed as a virtual cell and data
cell. I can understand that virtual cell is used to prevent discard bio
and non-discard bio targeting the same block from being processed at the
same time. I find it was added in commit
e8088073c9610af017fd47fddd104a2c3afb32e8 (dm thin: fix race between
simultaneous io and discards to same block), but I'm still confused
about the use of data cell.
The second question is the impact of virtual cell and data cell on IO
performance. If $data_block_size is large for example 1G, in multithread
fio test, most bio will be buffered in cell->bios list and then be
processed by worker thread asynchronously, even when there's no discard
bio. Thus the original parallel IO is processed by worker thread
serially now. As the number of fio test threads increase, the single
worker thread can easily get CPU 100%, and thus become the bottleneck of
the performance since dm-thin workqueue is ordered unbound.
Using an nvme SSD and fio (direct=1, ioengine=libaio, iodepth=128,
numjobs=4, rw=read, bs=4k), the bandwidth on bare nvme is 1589MiB/s. The
bandwidth on thin device is only 1274MiB/s, while the four fio threads
run at 200% CPU and the single worker thread is always runing at 100%
CPU. perf of worker thread showes that process_bio() consumes 86% of the
time.
Besides it seems that I can't send email to dm-devel@xxxxxxxxxx mailing
list.
Regards
Jeffle Xu
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