Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed

[Avi Kivity <avi@redhat.com>]

Ryan Harper wrote:
> I'd rather avoid any additional accounting overhead of a pool.

The accounting overhead is noise compared to copying hundreds of 
megabytes per second.


> If 4MB
> is a reasonable limit, lets make that the new max.  

The real max is the dma buffer size multiplied by the number of 
concurrent requests.  With a queue depth of 64, the buffers become 4 MB 
* 64 = 256 MB.  That can double the size of a small guest, and using 
just one disk, too.

> I can do some
> testing to see where we drop off on performance improvements.  We'd
> have a default buffer size (smaller than the previous 64, and now 128k
> buf size) that is used when we allocate scsi requests; scanning through
> send_command() provides a good idea of other scsi command buf usage; and
> on reads and writes, keep the capping logic we've had all along, but
> bump the max size up to something like 4MB -- or whatever tests results
> show as being ideal. 
>   

We know what the ideal is: dropping the scatter/gather buffer completely.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.