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Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path i
From: |
Ryan Harper |
Subject: |
Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed |
Date: |
Sun, 5 Oct 2008 18:06:56 -0500 |
User-agent: |
Mutt/1.5.6+20040907i |
* Anthony Liguori <address@hidden> [2008-10-04 17:28]:
> Ryan Harper wrote:
> >I'd rather avoid any additional accounting overhead of a pool. If 4MB
> >is a reasonable limit, lets make that the new max. 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.
> >
> >In all, it seems silly to worry about this sort of thing since the
> >entire process could be contained with process ulimits if this is really
> >a concern. Are we any more concerned that by splitting the requests
> >into many smaller requests that we're wasting cpu, pegging the
> >processor to 100% in some cases?
> >
>
> There are two concerns with allowing the guest to alloc arbitrary
> amounts of memory. The first is that QEMU is not written in such a way
> to be robust in the face of out-of-memory conditions so if we run out of
> VA space, an important malloc could fail and we'd fall over.
That is an understandable concern and I don't want to make matters
worse, even if the instability already exists in the code as-is. I think
I'd like to see this fail in practice before I'm really concerned. For
64-bit builds, is the VA space an issue?
>
> The second concern is that if a guest can allocate arbitrary amounts of
> memory, it could generate a swap storm. Unfortunately, AFAIK, Linux is
> not yet to a point where it can deal with swap fairness. Hopefully this
> is a limitation that the IO controller folks are taking into account.
Sure, but as I mentioned, the amount of memory it can allocate can
surely be controlled by the host system per-process ulimit no?
--
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
(512) 838-9253 T/L: 678-9253
address@hidden
- [Qemu-devel] [PATCH 2/4] Refactor lsi_do_command to queue read and write ops, (continued)
- [Qemu-devel] [PATCH 2/4] Refactor lsi_do_command to queue read and write ops, Ryan Harper, 2008/10/03
- [Qemu-devel] [PATCH 3/4] Refactor scsi-disk layer for queue'ing writes, Ryan Harper, 2008/10/03
- [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed, Ryan Harper, 2008/10/03
- Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed, Paul Brook, 2008/10/03
- Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed, Anthony Liguori, 2008/10/03
- Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed, Paul Brook, 2008/10/03
- Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed, Avi Kivity, 2008/10/04
- Message not available
- Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed, Ryan Harper, 2008/10/04
- Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed, Anthony Liguori, 2008/10/04
- Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed, Avi Kivity, 2008/10/05
- Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed,
Ryan Harper <=
- Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed, Avi Kivity, 2008/10/06
- Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed, Paul Brook, 2008/10/04
- Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed, Avi Kivity, 2008/10/05
Re: [Qemu-devel] [PATCH 0/4] Improve emulated scsi write performance, Ryan Harper, 2008/10/05