NAND Performance

The $199 Nexus 7 ships with 8GB of NAND on-board in the form of a standalone eMMC 4.41 package from Kingston. The single package storage solution features NAND and controller, although as you can guess there's no room for DRAM. The NAND in use is MLC although it's not clear whether we're looking at 2 or 3-bits per cell. This basic architecture shouldn't be all that unfamiliar to long time AnandTech readers - the cacheless design is exactly what plagued some of the early MLC SSDs for PCs a few years ago.

More recently users have been noticing very poor performance with ASUS Android tablets whenever there's substantial background IO going on, particularly multitasking while writes are taking place (e.g. downloading a torrent and browsing the web). The slowdowns range from multi-second long pauses to sluggish response time. The video below shows you what can happen on a Transformer Pad Infinity while downloading a bunch of large files in the background:

Even though downloading a large file is a largely sequential write operation, any other action performed while that download is in progress will effectively turn the IO stream from purely sequential to pseudo-random. Most of these eMMC and other cheap MLC NAND flash controllers are really glorified SD cards, primarily optimized for reading and writing large images as if they were used in a camera. What they're not designed for is to run and be used in a full blown multitasking OS. Some manufacturers seem to do a better job of picking their storage solution, and the Kingston eMMC in the Nexus 7 is faster than the Hynix e-NAND ASUS has used in previous tablets. Neither is perfect however. There's a tangible impact on simple multitasking if you're downloading a lot of files or installing apps in the background. Even operations in memory are quite negatively affected by background IO. Take a look at how SunSpider performance is hurt by a background file download:

SunSpider 9.1 Performance
  Default With 2MB/s Background Download
Google Nexus 7 (8GB) 1665.9 ms 1984.9 ms

To put all of this in perspective, I turned to Androbench, a storage benchmark that allows an ok amount of flexibility in configuring the test environment. At its default settings the benchmark varies a bit too much for my liking, but if you up the buffer sizes to 100MB it helps smooth out some of the variance.

Here we're looking at both sequential and random IO, the two sides we pay attention to in our SSD reviews as well. It's not clear to me whether or not Androbench is clamping the random IO to a particular range of addresses or not, but the results are telling nonetheless:

Androbench (100MB read/write)
  256KB Sequential Read 256KB Sequential Write 4KB Random Read 4KB Random Write
ASUS Transformer Pad Infinity (64GB) 15.9 MB/s 7.13 MB/s 4.90 MB/s 0.22 MB/s
Google Nexus 7 (8GB) 23.1 MB/s 8.43 MB/s 4.77 MB/s 0.32 MB/s
Samsung Galaxy Nexus (16GB) 26.8 MB/s 7.29 MB/s 7.46 MB/s 0.23 MB/s

Sequential read/write speed isn't bad, but it's the random write speed that's really a problem. We're talking about write speeds of a couple hundred KB/s. Remember what I said earlier about how multitasking can take an otherwise sequential IO stream and make it look fairly random? I suspect the low random write performance is one reason we're seeing significant slowdowns with background IO. Not all NAND controllers do well with concurrent reads and writes, which could be another contributing factor to poor performance.

Note that for light usage this isn't a problem. Similar to the first generation of affordable MLC SSDs for PCs, as long as you're doing a lot of reading you'll be ok. It's really for the heavier usage models that this is a problem. On a tablet however, simple background installation or downloading of files counts as heavy these days.

The real solution to this problem is to integrate better NAND flash controllers on-board, or even onto the SoC itself. Tablet makers are still mostly focused on cost cutting, but eventually we'll see real SSDs with good controllers in these things. Windows 8 tablets, at least the high end ones due out later this year will be among the first to incorporate real SSDs.

WiFi Performance, NFC

The Nexus 7 includes support for single spatial stream 802.11b/g/n on 2.4 GHz only. That ends up being a PHY rate of 65 Mbps (single spatial stream with no SGI). ASUS has been using AzureWave modules for basically every single one of its tablets, and the Nexus 7 is no exception. Thanks to the FCC test reports, you can see an AW-NH665 module alongside the new NXP PN65 NFC controller. The AW-NH665 includes a BCM4330 inside - remember there are different vendors who will sell you a completed module.


NXP PN65 NFC, AzureWave AW-NH665 (Source: FCC Internal Photos)

If you're looking for a cost cutting measure, the lack of 5 GHz WLAN support is most definitely one. In fact, the lack of 5 GHz WLAN made for probably one of my most memorably hilarious Google I/O experiences, since the event had multiple signs noting that only devices with 5 GHz WLAN would work well (or at all - 2.4 GHz is unuseable at any major event) on the conference WiFi. To mitigate this, Google hooked up USB OTG to MacBook Air ethernet adapters for its Nexus 7 demo stands, which I of course hijacked for my Galaxy Nexus at one point. More and more, having 5 GHz WLAN in a tablet is an expectation, and soon we'll even start seeing 2x2:2 antenna configurations.

The resulting performance is pretty par for the course when it comes to WiFi speeds among single spatial stream 2.4 GHz only devices. I tested using iperf the same way I do smartphones. 

WiFi Performance - iPerf

Unlike some of the other ASUS tablets of note, there are no reception issues with the Nexus 7 either. I have no problem getting good reception and see the Nexus 7 fall off the network where I expect it to in my area. I think it's worth being explicit about where the antennas are located since wireless connectivity issues have been a recurring issue for just about every product. 


BT/WLAN (red), NFC (blue), GPS (green)

I've gone ahead and marked where the antennas are for BT/WLAN, NFC, and GPS on the case from the FCC teardown photos. Knowing exactly where the NFC coil antenna is located is actually hugely important for correctly positioning the device for beaming or reading tags. It ends up being right behind the "nexus" recessed lettering on the back, but until I saw the antenna in the FCC teardowns I wasn't completely sure. With a smartphone the size makes it relatively forgiving, but tablets are less forgiving about positioning not being exact thanks to the larger size. 

GPS

The Nexus 7 uses a monolithic Broadcom BCM4751 GPS receiver, which we've seen in other devices like the iPad 2. This isn't the absolute newest broadcom GPS/GNSS, and again I'm sure ASUS was able to secure great pricing for this since the newer GNSS (GPS+GLONASS) modules are making their way into devices right now. 

That said, I had no problems with GPS locking quickly from cold and warm starts. I walked around San Francisco with the Nexus 7 using Google Maps (tethered to an SGS3) and saw pretty good performance in the urban canyon scenario. With no occlusions to the sky, performance is excellent, and locks indoors are possible too. 

 

GPU Performance Battery Life
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  • tuxRoller - Friday, July 27, 2012 - link

    One more thing: the lag that happened when there was high io suggests that android isn't properly using the softirq for io that the preempt kernel provides.
    As for the cheap nand, I don't see why the oems couldn't use raw nand and then use a file system like nilfs or one of the others to let the kernel properly handle the io.
    Reply
  • ssddaydream - Friday, July 27, 2012 - link

    Interestingly, the Trinity kernel seems to improve IO throughput when used in conjunction with a certain script (TKT app runs the script when opened, I believe.) Reply
  • tipoo - Friday, July 27, 2012 - link

    Yeah, I never got why it could be mostly smooth but going to a that blank search screen always lagged. Also similar to this if an app is writing to NAND it can take a while to get back to the home screen. Reply
  • TekDemon - Thursday, July 26, 2012 - link

    To be fair the Kindle still has one other advantage as an e-Reader, you get to borrow a free book per month if you're a Prime member, in addition to the free video streaming support. So here it's really Amazon's ecosystem that's the Kindle's big feature-if you've bought into their ecosystem it'll give you quite a bit more content and they have a $39 per year student Prime (that you get after the expired Student trial) that does let you access videos/books.

    Of course if you're into the Amazon ecosystem I'd suspect that you're waiting on the Fire 2.
    Reply
  • SantaAna12 - Thursday, July 26, 2012 - link

    I think Gabe Newells comments regarding Apples controlling access is applicable to this device and Google. Google wants us on their cloud to sell us content.....its one of their business models. Amazon does the same.....but you can be smart and buy the CD. I think the more we talk about these issues the better......as knowledge is power. For me....this device doesn't work because of the lack of memory....and the known lack of sound quality. Reply
  • milli - Friday, July 27, 2012 - link

    I feel you're ignoring the Playbook comparison a bit too much in this review. I've just bought a new Playbook 32GB for $175. Same 7" screen size (lower res though), OMAP4, bigger battery, better cam (front & back), ....
    Playbook OS is very usable since version 2.0. I use mine strictly professionally and at that, it's probably better than iOS en Android. Version 2.1 is around the corner and 3.0 around new years.
    Maybe not the best option if you're an app addict but the amazing multitasking combined with what most call the best mobile browsing experience, made this a no brainer for me. The 'Print to Go' feature is very handy.
    Reply
  • kenyee - Friday, July 27, 2012 - link

    I have a Playbook too (earned it for porting an Android app).
    Playbook as muchhhhh better speakers.

    Nexus 7 is what I grab for if I'm reading, etc. It's lighter than the Kindle and Nook so it's a lot more comfortable to hold for extended periods of time.
    And no comparison to any 10" tablet...not sure what he reviewer is thinking...the 7" ones have a perfect use case for reading and playing games. The 10" ones might be better for web surfing for a little while until you get tired of holding it...
    Reply
  • joshv - Friday, July 27, 2012 - link

    One of my frustrations with the Fire (one of many) is the WiFi reconnect when you wake the device. It would frequently take 20-30 seconds, and that's a ton of time for a casual use device. On the other hand the iPad just seems to be always on the Internet. Pick it up, launch the browser, the Internet is on. I know that can't actually be the case, as it would decimate battery life, but the iPad somehow manages the WiFi connection almost perfectly.

    So, I am wondering how the Nexus 7 performs in this regard.
    Reply
  • Impulses - Friday, July 27, 2012 - link

    If it's anything like ASUS' other tablets (minus the Prime & it's signal strength issues) then it should be pretty seamless. I've rarely ever been held back while waiting for my original Transformer to connect, and I actually have it set to go into airplane mode when the screen is off, for maximum battery life... So it's doing a cold reconnect every time and it's usually good to go before I tap the browser icon.

    I'm pretty sure it reconnects faster than my Krait phone, tho the handover from 3G to Wifi is probably never given priority.

    FWIW, the TF loses about half a percentage of battery life like that when sleeping. You can set it to always remain connected if you wanna receive live notifications or whatever but it'll obviously chew threw battery life much faster (probably twice as fast while idle, so you'd need to charge it every night or every other night at best).
    Reply
  • kenyee - Friday, July 27, 2012 - link

    It defaults to keeping wifi on even if the screen is off...and no problem w/ the batteries Reply

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