Vladislav Bolkhovitin wrote:
- Scst has some performance advantages over stgt, at least, on hardware targets, because it allows internal handling in SIRQ context as well as doesn't need user space program, so it eliminates additi switches (at least 3 per command for WRITEs and 2 per command for reads plus switches to user space daemon, probably, 2 per command). 5 context switches per command looks too much for me, especially considering how little work is done on each context. It means ~15000 CS/sec on regular 200Mb/sec link with 64K block size. Additionally, kernel only commands execution allows direct access to page cache, which is GREAT performance improvement, because in architecture with user space daemon the data should be copied several times between kernel and user space. Or, do I miss anything?
Userspace only occurs for READ/WRITEs. So for REPORT_LUNS, TUR, etc where performance is not a factor.
Also is the page cache comment in reference to us using the page cache for our reads and writes or I am not sure why you wrote that if you do not do it right now.
From other side, stgt has not too much advantages over scst.
Hey, we just started and have not had too much time.
Actually, we would greatly appreciate if Mike or Christoph will tell us what is so wrong in scst on their opinion, so they started they own new project. (I'm not considering motivation like "I want to make my own in any case" seriously). Is scst too complicated? Do you think stgt will be simpler with all those features implemented?
Didn't we go over this? To get SCST ready for mainline we would have had a large cleanup effort. I started this and sent you a patch to begin the cleanup. In the end some of the scsi people liked the idea of throwing the non-read/write command to userspace and to do this we just decided to start over but I have been cutting and pasting your code and cleaning it up as I add more stuff.
Simmer down :) If you had just gotton your code ready when christoph asked a year ago we would never have had this problem and I would be sending you patches to remove the scsi_request usage as we speak.
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