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Re: [win-pv-devel] Question about XENVBD_MAX_REQUESTS_PER_SRB



Stortport will ignore any max request size below 16 sectors per request, so setting XENVBD_MAX_REQUESTS_PER_SRB to 1 (11 sectors per SRB) will unfortunately not simplify things too much.

16 was chosen based on fairly crude performance tests with a single page ring when xenvbd was originally written. There is 32 blkif requests per page of ring, so going above half this starts to prevent responses entering the ring and creates synchronization problems.

It should be possible to use storport’s queues to queue SRBs and only prepare/submit one ring’s worth of requests, to reduce the number of queues in xenvbd.




From: win-pv-devel [mailto:win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf Of Lai, Ethan
Sent: 25 March 2016 23:03
To: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
Subject: [win-pv-devel] Question about XENVBD_MAX_REQUESTS_PER_SRB




I’m curious that why XENVBD_MAX_REQUESTS_PER_SRB is set to 16. Is it because that XENVBD_MAX_RING_PAGES is also 16?


If the XENVBD_MAX_REQUESTS_PER_SRB is set to 1, then there will be one to one mapping between srb and xenvbd request, which make things simpler. i.e. count in SrbExt would be saved, and it is possible to reuse storport’s queue to hold prepared requests.




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