Re: [PATCH] vhost: support upto 509 memory regions
On Tue, Feb 17, 2015 at 4:32 AM, Michael S. Tsirkin m...@redhat.com wrote: On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote: On 17/02/2015 10:02, Michael S. Tsirkin wrote: Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509 to match KVM_USER_MEM_SLOTS fixes issue for vhost-net. Signed-off-by: Igor Mammedov imamm...@redhat.com This scares me a bit: each region is 32byte, we are talking a 16K allocation that userspace can trigger. What's bad with a 16K allocation? It fails when memory is fragmented. How does kvm handle this issue? It doesn't. Paolo I'm guessing kvm doesn't do memory scans on data path, vhost does. qemu is just doing things that kernel didn't expect it to need. Instead, I suggest reducing number of GPA-HVA mappings: you have GPA 1,5,7 map them at HVA 11,15,17 then you can have 1 slot: 1-11 To avoid libc reusing the memory holes, reserve them with MAP_NORESERVE or something like this. This works beautifully when host virtual address bits are more plentiful than guest physical address bits. Not all architectures have that property, though. We can discuss smarter lookup algorithms but I'd rather userspace didn't do things that we then have to work around in kernel. -- MST -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] vhost: support upto 509 memory regions
On Tue, Feb 17, 2015 at 04:53:45PM -0800, Eric Northup wrote: On Tue, Feb 17, 2015 at 4:32 AM, Michael S. Tsirkin m...@redhat.com wrote: On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote: On 17/02/2015 10:02, Michael S. Tsirkin wrote: Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509 to match KVM_USER_MEM_SLOTS fixes issue for vhost-net. Signed-off-by: Igor Mammedov imamm...@redhat.com This scares me a bit: each region is 32byte, we are talking a 16K allocation that userspace can trigger. What's bad with a 16K allocation? It fails when memory is fragmented. How does kvm handle this issue? It doesn't. Paolo I'm guessing kvm doesn't do memory scans on data path, vhost does. qemu is just doing things that kernel didn't expect it to need. Instead, I suggest reducing number of GPA-HVA mappings: you have GPA 1,5,7 map them at HVA 11,15,17 then you can have 1 slot: 1-11 To avoid libc reusing the memory holes, reserve them with MAP_NORESERVE or something like this. This works beautifully when host virtual address bits are more plentiful than guest physical address bits. Not all architectures have that property, though. AFAIK this is pretty much a requirement for both kvm and vhost, as we require each guest page to also be mapped in qemu memory. We can discuss smarter lookup algorithms but I'd rather userspace didn't do things that we then have to work around in kernel. -- MST -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] vhost: support upto 509 memory regions
On Tue, 17 Feb 2015 14:29:31 +0100 Michael S. Tsirkin m...@redhat.com wrote: On Tue, Feb 17, 2015 at 02:11:37PM +0100, Paolo Bonzini wrote: On 17/02/2015 13:32, Michael S. Tsirkin wrote: On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote: On 17/02/2015 10:02, Michael S. Tsirkin wrote: Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509 to match KVM_USER_MEM_SLOTS fixes issue for vhost-net. Signed-off-by: Igor Mammedov imamm...@redhat.com This scares me a bit: each region is 32byte, we are talking a 16K allocation that userspace can trigger. What's bad with a 16K allocation? It fails when memory is fragmented. If memory is _that_ fragmented I think you have much bigger problems than vhost. I'm guessing kvm doesn't do memory scans on data path, vhost does. It does for MMIO memory-to-memory writes, but that's not a particularly fast path. KVM doesn't access the memory map on fast paths, but QEMU does, so I don't think it's beyond the expectations of the kernel. QEMU has an elaborate data structure to deal with that. For example you can use a radix tree (not lib/radix-tree.c unfortunately), and cache GVA-HPA translations if it turns out that lookup has become a hot path. All vhost lookups are hot path. The addressing space of x86 is in practice 44 bits or fewer, and each slot will typically be at least 1 GiB, so you only have 14 bits to dispatch on. It's probably possible to only have two or three levels in the radix tree in the common case, and beat the linear scan real quick. Not if there are about 6 regions, I think. When memslots where increased to 509 and look up of them was replaced on binary search results were on par with linear search for a default 13 memslots VM. Adding LRU cache helped to shave ~40% of cycles for sequential lookup workloads. The radix tree can be tuned to use order-0 allocations, and then your worries about fragmentation go away too. Paolo Increasing the number might be reasonable for workloads such as nested virt. But depending on this in userspace when you don't have to is not a good idea IMHO. -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] vhost: support upto 509 memory regions
On 17/02/2015 16:02, Igor Mammedov wrote: Not if there are about 6 regions, I think. When memslots where increased to 509 and look up of them was replaced on binary search results were on par with linear search for a default 13 memslots VM. Adding LRU You mean MRU. :) cache helped to shave ~40% of cycles for sequential lookup workloads. It's a bit different for vhost because you can have up to four things being looked up at the same time: - the s/g list that will end up in the skb - the avail/used ring - the virtio buffers - the virtio indirect buffers So you probably need multiple MRU caches. But yes, MRU can help a lot. Paolo -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] vhost: support upto 509 memory regions
On Tue, Feb 17, 2015 at 02:11:37PM +0100, Paolo Bonzini wrote: On 17/02/2015 13:32, Michael S. Tsirkin wrote: On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote: On 17/02/2015 10:02, Michael S. Tsirkin wrote: Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509 to match KVM_USER_MEM_SLOTS fixes issue for vhost-net. Signed-off-by: Igor Mammedov imamm...@redhat.com This scares me a bit: each region is 32byte, we are talking a 16K allocation that userspace can trigger. What's bad with a 16K allocation? It fails when memory is fragmented. If memory is _that_ fragmented I think you have much bigger problems than vhost. I'm guessing kvm doesn't do memory scans on data path, vhost does. It does for MMIO memory-to-memory writes, but that's not a particularly fast path. KVM doesn't access the memory map on fast paths, but QEMU does, so I don't think it's beyond the expectations of the kernel. QEMU has an elaborate data structure to deal with that. For example you can use a radix tree (not lib/radix-tree.c unfortunately), and cache GVA-HPA translations if it turns out that lookup has become a hot path. All vhost lookups are hot path. The addressing space of x86 is in practice 44 bits or fewer, and each slot will typically be at least 1 GiB, so you only have 14 bits to dispatch on. It's probably possible to only have two or three levels in the radix tree in the common case, and beat the linear scan real quick. Not if there are about 6 regions, I think. The radix tree can be tuned to use order-0 allocations, and then your worries about fragmentation go away too. Paolo Increasing the number might be reasonable for workloads such as nested virt. But depending on this in userspace when you don't have to is not a good idea IMHO. -- MST -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] vhost: support upto 509 memory regions
On 17/02/2015 14:29, Michael S. Tsirkin wrote:
On Tue, Feb 17, 2015 at 02:11:37PM +0100, Paolo Bonzini wrote:
On 17/02/2015 13:32, Michael S. Tsirkin wrote:
On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote:
On 17/02/2015 10:02, Michael S. Tsirkin wrote:
Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509
to match KVM_USER_MEM_SLOTS fixes issue for vhost-net.
Signed-off-by: Igor Mammedov imamm...@redhat.com
This scares me a bit: each region is 32byte, we are talking
a 16K allocation that userspace can trigger.
What's bad with a 16K allocation?
It fails when memory is fragmented.
If memory is _that_ fragmented I think you have much bigger problems
than vhost.
I'm guessing kvm doesn't do memory scans on data path, vhost does.
It does for MMIO memory-to-memory writes, but that's not a particularly
fast path.
KVM doesn't access the memory map on fast paths, but QEMU does, so I
don't think it's beyond the expectations of the kernel.
QEMU has an elaborate data structure to deal with that.
It's not elaborate, it's just a radix tree. The complicated part is
building the flat view and computing what changed in the memory map, but
none of this would have to be done in vhost. vhost gets the flat memory
map in VHOST_SET_MEM_TABLE.
A lookup is basically:
#define LOG_TRIE_WIDTH (PAGE_SHIFT - LOG_BITS_PER_LONG)
unsigned long node_val = (unsigned long) trie_root;
/* log of highest valid address in the memory map */
if (addr (-1U vhost_address_space_bits))
return NULL;
addr = 64 - vhost_address_space_bits;
do {
struct memmap_trie_node *node;
unsigned i = addr (64 - LOG_TRIE_WIDTH);
addr = addr LOG_TRIE_WIDTH;
node = (struct memmap_trie_node *) (node_val - 1);
node_val = (unsigned long) node[i];
} while (node_val 1);
return (struct vhost_mem_slot *)node_val;
bit 0: 0 if leaf
if leaf:
bits 1-63: pointer to mem table entry
if not leaf:
bits 1-63: pointer to next level
For example you
can use a radix tree (not lib/radix-tree.c unfortunately), and cache
GVA-HPA translations if it turns out that lookup has become a hot path.
All vhost lookups are hot path.
What % is lookup vs the networking stuff? Also, adding a simple MRU
cache might make lookups less prominent in the profile.
The addressing space of x86 is in practice 44 bits or fewer, and each
slot will typically be at least 1 GiB, so you only have 14 bits to
dispatch on. It's probably possible to only have two or three levels
in the radix tree in the common case, and beat the linear scan real quick.
Not if there are about 6 regions, I think.
It depends on many factors including branch prediction, MRU cache hits, etc.
Increasing the number might be reasonable for workloads such as nested
virt.
Why does nested virt matter?
Paolo
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Re: [PATCH] vhost: support upto 509 memory regions
On Tue, 17 Feb 2015 13:32:12 +0100 Michael S. Tsirkin m...@redhat.com wrote: On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote: On 17/02/2015 10:02, Michael S. Tsirkin wrote: Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509 to match KVM_USER_MEM_SLOTS fixes issue for vhost-net. Signed-off-by: Igor Mammedov imamm...@redhat.com This scares me a bit: each region is 32byte, we are talking a 16K allocation that userspace can trigger. What's bad with a 16K allocation? It fails when memory is fragmented. How does kvm handle this issue? It doesn't. Paolo I'm guessing kvm doesn't do memory scans on data path, vhost does. qemu is just doing things that kernel didn't expect it to need. Instead, I suggest reducing number of GPA-HVA mappings: you have GPA 1,5,7 map them at HVA 11,15,17 then you can have 1 slot: 1-11 To avoid libc reusing the memory holes, reserve them with MAP_NORESERVE or something like this. Lets suppose that we add API to reserve whole memory hotplug region with MAP_NORESERVE and passed it as memslot to KVM. Then what will happen to guest accessing not really mapped region? This memslot will also be passed to vhost as region, is it really ok? I don't know what else it might break. As alternative: we can filter out hotplugged memory and vhost will continue to work with only initial memory. So question is id we have to tell vhost about hotplugged memory? We can discuss smarter lookup algorithms but I'd rather userspace didn't do things that we then have to work around in kernel. -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] vhost: support upto 509 memory regions
On 17/02/2015 15:44, Igor Mammedov wrote: As alternative: we can filter out hotplugged memory and vhost will continue to work with only initial memory. So question is id we have to tell vhost about hotplugged memory? Yes, I think so. Paolo -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] vhost: support upto 509 memory regions
On 17/02/2015 10:02, Michael S. Tsirkin wrote: Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509 to match KVM_USER_MEM_SLOTS fixes issue for vhost-net. Signed-off-by: Igor Mammedov imamm...@redhat.com This scares me a bit: each region is 32byte, we are talking a 16K allocation that userspace can trigger. What's bad with a 16K allocation? How does kvm handle this issue? It doesn't. Paolo -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] vhost: support upto 509 memory regions
On Fri, Feb 13, 2015 at 03:49:59PM +, Igor Mammedov wrote:
since commit
1d4e7e3 kvm: x86: increase user memory slots to 509
it became possible to use a bigger amount of memory
slots, which is used by memory hotplug for
registering hotplugged memory.
However QEMU aborts if it's used with more than ~60
pc-dimm devices and vhost-net since host kernel
in module vhost-net refuses to accept more than 65
memory regions.
Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509
to match KVM_USER_MEM_SLOTS fixes issue for vhost-net.
Signed-off-by: Igor Mammedov imamm...@redhat.com
This scares me a bit: each region is 32byte, we are talking
a 16K allocation that userspace can trigger.
How does kvm handle this issue?
---
drivers/vhost/vhost.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 2ee2826..ecbd7a4 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -29,7 +29,7 @@
#include vhost.h
enum {
- VHOST_MEMORY_MAX_NREGIONS = 64,
+ VHOST_MEMORY_MAX_NREGIONS = 509,
VHOST_MEMORY_F_LOG = 0x1,
};
--
1.8.3.1
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Re: [PATCH] vhost: support upto 509 memory regions
On 17/02/2015 13:32, Michael S. Tsirkin wrote: On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote: On 17/02/2015 10:02, Michael S. Tsirkin wrote: Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509 to match KVM_USER_MEM_SLOTS fixes issue for vhost-net. Signed-off-by: Igor Mammedov imamm...@redhat.com This scares me a bit: each region is 32byte, we are talking a 16K allocation that userspace can trigger. What's bad with a 16K allocation? It fails when memory is fragmented. If memory is _that_ fragmented I think you have much bigger problems than vhost. I'm guessing kvm doesn't do memory scans on data path, vhost does. It does for MMIO memory-to-memory writes, but that's not a particularly fast path. KVM doesn't access the memory map on fast paths, but QEMU does, so I don't think it's beyond the expectations of the kernel. For example you can use a radix tree (not lib/radix-tree.c unfortunately), and cache GVA-HPA translations if it turns out that lookup has become a hot path. The addressing space of x86 is in practice 44 bits or fewer, and each slot will typically be at least 1 GiB, so you only have 14 bits to dispatch on. It's probably possible to only have two or three levels in the radix tree in the common case, and beat the linear scan real quick. The radix tree can be tuned to use order-0 allocations, and then your worries about fragmentation go away too. Paolo -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] vhost: support upto 509 memory regions
On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote: On 17/02/2015 10:02, Michael S. Tsirkin wrote: Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509 to match KVM_USER_MEM_SLOTS fixes issue for vhost-net. Signed-off-by: Igor Mammedov imamm...@redhat.com This scares me a bit: each region is 32byte, we are talking a 16K allocation that userspace can trigger. What's bad with a 16K allocation? It fails when memory is fragmented. How does kvm handle this issue? It doesn't. Paolo I'm guessing kvm doesn't do memory scans on data path, vhost does. qemu is just doing things that kernel didn't expect it to need. Instead, I suggest reducing number of GPA-HVA mappings: you have GPA 1,5,7 map them at HVA 11,15,17 then you can have 1 slot: 1-11 To avoid libc reusing the memory holes, reserve them with MAP_NORESERVE or something like this. We can discuss smarter lookup algorithms but I'd rather userspace didn't do things that we then have to work around in kernel. -- MST -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
